Content of table (longtable)overlapping with content after the table

Question

My lower part of the longtable is overlapping with the content written below the table. Second problem is the caption is not placed in the centre. I have used the longtable package and table is 6 pages long

       \documentclass[journal]{IEEEtran}
         \usepackage{graphicx}
        % \usepackage{subfigure}
          \usepackage{subfig}
    \usepackage{caption}
      \usepackage{lipsum}
      \usepackage{tablefootnote}
     \usepackage{hyperref}
       \usepackage{amsmath, amssymb}
       \usepackage{csquotes}
         \usepackage[demo,
            export]{adjustbox}
         \usepackage{stfloats}
         \usepackage{makecell,
            ltablex}    % new
        \keepXColumns
         % \raggedbottom
        % \usepackage{parskip} 
       \setcellgapes{3pt}
     \usepackage{enumitem}
         \usepackage{longtable}
         \usepackage{siunitx}

       % \usepackage[T1]{fontenc}
         % \usepackage[utf8]{inputenc}
         \setlength{\LTpre}{0pt}
        \setlength{\LTpost}{0pt}
         %%
          %% \BibTeX command to typeset BibTeX logo in the docs

          \setlength\intextsep{\glueexpr\intextsep/2\relax}
           \AtBeginDocument{%
          \providecommand\BibTeX{{%
         \normalfont B\kern-0.5em{\scshape i\kern-0.25em b}\kern-0.8em\TeX}}}

      % \maketitle
            \begin{document}  


         \section{ABCD}
        Currently, JPEG and JPEG2000 [28] are the international
       standards for image compression, and h. 265/HEVC [29]
        is the latest video compression standards, which can also
       implement the intra-frame coding as image compression.
      JPEG is the earliest proposed image compression standard
        based on discrete cosine transform (DCT) [30]. After the
        image is decomposed by DCT, the high-frequency coefficients are 
          dramatically quantized and compressed to realize
    data compression. However, each coefficient after DCT is
     related to all the pixels of the whole image; thus, it requires a
        large amount of computation. Therefore, block segmentation
      is carried out before implementing the subsequent compression, which leads 
       to some block artifacts [31] under low bit
            rate. Compared with JPEG, JPEG2000 is based on DWT.
        Because the length of the base function of the DWT is variable, the 
     coefficients after DWT only reflect some local features of the input image, 
      showing good local characteristics.
       Currently, JPEG and JPEG2000 [28] are the international
        standards for image compression, and h. 265/HEVC [29]
     is the latest video compression standards, which can also
    implement the intra-frame coding as image compression.
       JPEG is the earliest proposed image compression standard
        based on discrete cosine transform (DCT) [30]. After the
     image is decomposed by DCT, the high-frequency coefficients are dramatically 
       quantized and compressed to realize
       data compression. However, each coefficient after DCT is
        related to all the pixels of the whole image; thus, it requires a
     large amount of computation. Therefore, block segmentation
      is carried out before implementing the subsequent compression, which leads 
       to some block artifacts [31] under low bit
       rate. Compared with JPEG, JPEG2000 is based on DWT.
           Because the length of the base function of the DWT is variable, the 
       coefficients after DWT only reflect some local features of the input 
         image, showing good local characteristics


         \begin{center}
         \scriptsize
            \onecolumn
           \begin{longtable}{|p{1cm}|p{0.5cm}|p{1.5cm}|p{3cm}|p{2cm}|p{8cm}|}

          \caption{ABCDDDDDDDDDDDDDDDDDDDDD}\\
        \hline
          \thead{\textbf{ABCD}} 
          & \thead{\textbf{ABC}}     
            & \thead{\textbf{ABCD}} 

            % &  \thead{\specialcell{\textbf{Loss}} \\\ 
        \specialcell{\textbf{Function}}}
          & \thead{\specialcell{\textbf{ABCD} \\\ \specialcell{\textbf{ABCD}}} }   
        & \thead{\specialcell{\textbf{ABCD} \\\ \& \specialcell{\textbf{ABCD}}} }  
           & \thead{\textbf{ABCD}}     \\
           % \textbf{First entryFirst entry} & \textbf{Second entryFirst entry} & 
        \textbf{Third entryFirst entry} & \textbf{Fourth entryFirst entry} \\
      \hline
        \endfirsthead
        \multicolumn{6}{c}%
           {\tablename\ \thetable\ -- \textit{Continued from previous page}} \\
            \hline
          \thead{\textbf{Paper}} 
       & \thead{\textbf{DNN}}     
      & \thead{\textbf{Train/Test}} 

        % &  \thead{\specialcell{\textbf{Loss}} \\\ 
      \specialcell{\textbf{Function}}}
             & \thead{\specialcell{\textbf{Performance} \\\ 
        \specialcell{\textbf{Measures}}} }   
        & \thead{\specialcell{\textbf{Complexity} \\\ \& 
          \specialcell{\textbf{Runtime}}} }  
           & \thead{\textbf{Critical Findings/Remarks}}     \\
          % \textbf{First entry} & \textbf{Second entry} & \textbf{Third entry} & 
           \textbf{Fourth entry} \\
            \hline
         \endhead
      \hline 
           \multicolumn{6}{r}{\textit{Continued on next page}} \\
        \endfoot
         \hline
         \endlastfoot

        \textbf{vvvv}  2016 & A  & a & a &  
           \begin{minipage}[t]{\linewidth}
            \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
                \item a
          \item a
          \end{itemize} 
           \end{minipage} 

            & \begin{minipage}[t]{\linewidth}
           \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
               \item a
              \item a
              \item a
             \end{itemize}           
                \end{minipage} \\

           \textbf{vvvv}  2016 & A  & a & a &  
          \begin{minipage}[t]{\linewidth}
            \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
             \item a
             \item a
            \end{itemize} 
                 \end{minipage} 

           & \begin{minipage}[t]{\linewidth}
      \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
         \item a
            \item a
             \item a
           \end{itemize}           
           \end{minipage} \\




        \textbf{vvvv}  2016 & A  & a & a &  
        \begin{minipage}[t]{\linewidth}
            \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
      \item a
         \item a
      \end{itemize} 
          \end{minipage} 

             & \begin{minipage}[t]{\linewidth}
          \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
              \item a
             \item a
                 \item a
         \end{itemize}           
               \end{minipage} \\

            \textbf{vvvv}  2016 & A  & a & a &  
             \begin{minipage}[t]{\linewidth}
              \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
           \item a
            \item a
            \end{itemize} 
             \end{minipage} 

           & \begin{minipage}[t]{\linewidth}
         \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
         \item a
           \item a
             \item a
             \end{itemize}           
          \end{minipage} \\

         \textbf{vvvv}  2016 & A  & a & a &  
          \begin{minipage}[t]{\linewidth}
             \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
               \item a
              \item a
           \end{itemize} 
              \end{minipage} 

            & \begin{minipage}[t]{\linewidth}
           \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
             \item a
        \item a
            \item a
          \end{itemize}           
              \end{minipage} \\

         \textbf{vvvv}  2016 & A  & a & a &  
           \begin{minipage}[t]{\linewidth}
             \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
            \item a
           \item a
        \end{itemize} 
               \end{minipage} 

          & \begin{minipage}[t]{\linewidth}
          \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
          \item a
             \item a
             \item a
         \end{itemize}           
              \end{minipage} \\

        \textbf{vvvv}  2016 & A  & a & a &  
         \begin{minipage}[t]{\linewidth}
          \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
        \item a
     \item a
          \end{itemize} 
          \end{minipage} 

         & \begin{minipage}[t]{\linewidth}
         \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
         \item a
           \item a
          \item a
            \end{itemize}           
              \end{minipage} \\

          \textbf{vvvv}  2016 & A  & a & a &  
       \begin{minipage}[t]{\linewidth}
          \begin{itemize}[topsep=-0.5cm,leftmargin=0.2cm]
          \item a
             \item a
          \end{itemize} 
            \end{minipage} 




         \hline

           \end{longtable}
         \twocolumn
         \end{center}


       \section{DCBA}
    Currently, JPEG and JPEG2000 [28] are the international
                        standards for image compression, and h. 265/HEVC [29]
      is the latest video compression standards, which can also
   implement the intra-frame coding as image compression.
     JPEG is the earliest proposed image compression standard
         based on discrete cosine transform (DCT) [30]. After the
       image is decomposed by DCT, the high-frequency coefficients are 
        dramatically quantized and compressed to realize
          data compression. However, each coefficient after DCT is
        related to all the pixels of the whole image; thus, it requires a
      large amount of computation. Therefore, block segmentation
      is carried out before implementing the subsequent compression, which leads 
            to some block artifacts [31] under low bit
     rate. Compared with JPEG, JPEG2000 is based on DWT.
    Because the length of the base function of the DWT is variable, the 
         coefficients after DWT only reflect some local features of the input 
    image, showing good local characteristics.
      Currently, JPEG and JPEG2000 [28] are the international
      standards for image compression, and h. 265/HEVC [29]
      is the latest video compression standards, which can also
      implement the intra-frame coding as image compression.
     JPEG is the earliest proposed image compression standard
         based on discrete cosine transform (DCT) [30]. After the
        image is decomposed by DCT, the high-frequency coefficients are 
     dramatically quantized and compressed to realize
         data compression. However, each coefficient after DCT is
         related to all the pixels of the whole image; thus, it requires a
      large amount of computation. Therefore, block segmentation
       is carried out before implementing the subsequent compression, which leads 
         to some block artifacts [31] under low bit
      rate. Compared with JPEG, JPEG2000 is based on DWT.
       Because the length of the base function of the DWT is variable, the 
         coefficients after DWT only reflect some local features of the input image, 
            showing good local characteristics.




               \end{document}

Answer 1

Your MWE is very unclear, so just a proposition based on my answer mentioned in @leandriis's comment, what you can do:

• Manually split table onto two parts. The first had to have [b] placement option, the second [t]. Between them had not to be any space. This action exploit `
• Insert table close to beginning of page so, that the first part of table will have sufficient space on page or make first part of table small enough that will appear on the same page where is inserted.
• Use tabularx table
• For lists inside table set itemize by help of etoolbox:

\usepackage{etoolbox}                       %
\AtBeginEnvironment{tabularx}{%
\setlist[itemize]{nosep,
                 leftmargin=*,
                 label=\textbullet,
                 before=\begin{minipage}[t]{\linewidth}, % <---
                 after=\end{minipage}                    % <---
                 }  }

• for rules use rules defined in the booktabs package and remove all vertical lines

An example of your table (as far as Was able decipher your table code) can be:

\documentclass[journal]{IEEEtran}
%\usepackage{graphicx}
\usepackage{subfig}
\usepackage{caption}
\usepackage{tablefootnote}
\usepackage{amsmath, amssymb}
\usepackage{csquotes}
\usepackage[demo,
            export]{adjustbox} % it load graphicx too
\usepackage{stfloats}
\usepackage{booktabs, makecell, tabularx}   % changed
\renewcommand\theadfont{\small\bfseries}    % new
\renewcommand\theadgape{}                   % new
\setcellgapes{3pt}                          % new
\usepackage{siunitx}

\usepackage{enumitem}
\usepackage{etoolbox}                       %
\AtBeginEnvironment{tabularx}{%
\setlist[itemize]{nosep,
                 leftmargin=*,
                 label=\textbullet,
                 before=\begin{minipage}[t]{\linewidth}, % <---
                 after=\end{minipage}                    % <---
                 }  }

\usepackage{hyperref}
\usepackage{lipsum}

\begin{document}
    \section{ABCD}
Currently, JPEG and JPEG2000 [28] are the international standards for image compression, and h. 265/HEVC [29] is the latest video compression standards, which can also implement the intra-frame coding as image compression. JPEG is the earliest proposed image compression standard based on discrete cosine transform (DCT) [30]. After the image is decomposed by DCT, the high-frequency coefficients are dramatically quantized and compressed to realize data compression. However, each coefficient after DCT is related to all the pixels of the whole image; thus, it requires a large amount of computation. Therefore, block segmentation is carried out before implementing the subsequent compression, which leads to some block artifacts [31] under low bit rate. Compared with JPEG, JPEG2000 is based on DWT. Because the length of the base function of the DWT is variable, the coefficients after DWT only reflect some local features of the input image, showing good local characteristics.
    \begin{table*}[b]
\footnotesize
\setcellgapes{3pt}
\makegapedcells
\caption{ABCDDDDDDDDDDDDDDDDDDDDD}                  
\begin{tabularx}{\linewidth}{@{} p{1cm} l p{1.5cm} p{3cm} p{2cm} X @{}}
    \toprule
\thead{ABCD}
    & \thead{ABC}
    & \thead{ABCD}
    & \thead{Function}
    & \thead{ABCD}
    & \thead{Critical Findings/Remarks}                     \\
    \midrule   
    & \multicolumn{3}{c}{\thead{Third entry First entry}}
    & \multicolumn{2}{c}{\thead{Fourth entry First entry}} \\
    \cmidrule{2-4}\cmidrule(l){5-6}
%%%% table body
\textbf{vvvv}  2016 
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}   
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
%    \hline
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
%    \hline
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
    \bottomrule
\multicolumn{6}{r}{\textit{Continued on the next page}} \\
\end{tabularx}
    \end{table*}
    \begin{table*}[t]
    \ContinuedFloat
\footnotesize
\setcellgapes{3pt}
\makegapedcells
\caption[]{-- \textit{Continued from previous page}}
\begin{tabularx}{\linewidth}{@{} p{1cm} l p{1.5cm} p{3cm} p{2cm} X @{}}
    \toprule
\thead{ABCD}
    & \thead{ABC}
    & \thead{ABCD}
    & \thead{Function}
    & \thead{ABCD}
    & \thead{Critical Findings/Remarks}                     \\
    \midrule
    & \multicolumn{3}{c}{\thead{Third entry First entry}}
    & \multicolumn{2}{c}{\thead{Fourth entry First entry}} \\
    \cmidrule{2-4}\cmidrule(l){5-6}
%%%% table body
\textbf{vvvv}  2016
    & B & C & D &   \begin{itemize}
                \item a
                \item a
                    \end{itemize}
                    &   \begin{itemize}
                    \item a
                    \item a
                    \item a
                        \end{itemize}   \\
    \bottomrule
\end{tabularx}
    \end{table*}

% this paragraph are moved after table that the first part of table has enough space page where it is inserted
Currently, JPEG and JPEG2000 [28] are the international standards for image compression, and h. 265/HEVC [29] is the latest video compression standards, which can also implement the intra-frame coding as image compression. JPEG is the earliest proposed image compression standard based on discrete cosine transform (DCT) [30]. After the image is decomposed by DCT, the high-frequency coefficients are dramatically quantized and compressed to realize data compression. However, each coefficient after DCT is related to all the pixels of the whole image; thus, it requires a large amount of computation. Therefore, block segmentation is carried out before implementing the subsequent compression, which leads to some block artifacts [31] under low bit rate. Compared with JPEG, JPEG2000 is based on DWT. Because the length of the base function of the DWT is variable, the coefficients after DWT only reflect some local features of the input image, showing good local characteristics.

\section{DCBA}
Currently, JPEG and JPEG2000 [28] are the international standards for image compression, and h. 265/HEVC [29] is the latest video compression standards, which can also implement the intra-frame coding as image compression. JPEG is the earliest proposed image compression standard based on discrete cosine transform (DCT) [30]. After the image is decomposed by DCT, the high-frequency coefficients are dramatically quantized and compressed to realize data compression. However, each coefficient after DCT is related to all the pixels of the whole image; thus, it requires a large amount of computation. Therefore, block segmentation is carried out before implementing the subsequent compression, which leads to some block artifacts [31] under low bit rate. Compared with JPEG, JPEG2000 is based on DWT. Because the length of the base function of the DWT is variable, the coefficients after DWT only reflect some local features of the input image, showing good local characteristics.

Currently, JPEG and JPEG2000 [28] are the international standards for image compression, and h. 265/HEVC [29] is the latest video compression standards, which can also implement the intra-frame coding as image compression. JPEG is the earliest proposed image compression standard based on discrete cosine transform (DCT) [30]. After the image is decomposed by DCT, the high-frequency coefficients are dramatically quantized and compressed to realize data compression. However, each coefficient after DCT is related to all the pixels of the whole image; thus, it requires a large amount of computation. Therefore, block segmentation is carried out before implementing the subsequent compression, which leads to some block artifacts [31] under low bit rate. Compared with JPEG, JPEG2000 is based on DWT. Because the length of the base function of the DWT is variable, the coefficients after DWT only reflect some local features of the input image, showing good local characteristics.
   \end{document}

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