Why does using multicolumn change the rest of my table?

Question

I am currently creating a threeparttable with longtable and the dcolumn package in order to align my numbers by ".". Under my caption I want to write a description of the table. I am following suggestions of previous threads and try to use the multicolumn functionality to solve that issue. But this does not work, because the rest of my table is suddenly moved. Does someone have a suggestion on how to resolve this?

My code:

\documentclass[
    fontsize=12pt, 
    twoside=off 
]{scrbook}
\usepackage{caption}
\usepackage[flushleft]{threeparttable}
\usepackage{longtable}
\usepackage{threeparttablex}
\usepackage{dcolumn}
\newcolumntype{C}[1]{>{\raggedright\arraybackslash}p{#1}}
\begin{document}
\begin{ThreePartTable}
\small
\renewcommand{\arraystretch}{0.75}
\begin{longtable}{@{\extracolsep{\fill}}ld{1.1}d{1.1}d{1.3}c}
\caption{Some very informative caption}
\label{Table:1} \
\multicolumn{5}{C{\textwidth}}{I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text}\
\toprule
\multicolumn{1}{@{\extracolsep{\fill}}l}{K} & \multicolumn{1}{c}{b} & \multicolumn{1}{c}{t} & \multicolumn{1}{c}{pval} & \multicolumn{1}{c}{R$^2$} \
\toprule
\endfirsthead
\multicolumn{5}{@{\extracolsep{\fill}}l}{{\bfseries \tablename\ \thetable{} (continued)}}\
\toprule
\multicolumn{1}{@{\extracolsep{\fill}}l}{K} & \multicolumn{1}{c}{b} & \multicolumn{1}{c}{t} & \multicolumn{1}{c}{pval} & \multicolumn{1}{c}{R$^2$} \
\toprule

\endhead
\midrule
\multicolumn{5}{r}{{Continued on next page}} \
\midrule
\endfoot
\bottomrule
\endlastfoot
\multicolumn{5}{@{\extracolsep{\fill}}l}{Panel A: Hi}\
\midrule
  10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
  10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
  10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
  10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
  10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
\end{longtable}
\end{ThreePartTable}
\end{document}

I want my table to look like this: (example without the multicolumn text)

But sadly i get this:

I tried multiple things such as using the xltabular package instead of the longtable package, specifying different "p{}" but none seem to work.

Answer 1

A solution with tabularray:

\documentclass[
    fontsize=12pt, 
    twoside=off 
    ]{scrbook}
\usepackage{tabularray}
\UseTblrLibrary{siunitx}
\sisetup{group-digits=false}
\UseTblrLibrary{booktabs}
\NewColumnType{A}[1][1.2]{Q[si={table-format=#1},c,co=1]}
\DefTblrTemplate{conthead-text}{default}{(continued)}
\SetTblrStyle{caption-tag}{font=\bfseries}
% Code from https://tex.stackexchange.com/a/634814/101651, thanks to L.J.R.
\ExplSyntaxOn
\NewDocumentCommand\FirstOfTwo{m}{
  \exp_last_unbraced:Ne \use_i:nn { #1 }
}
\NewDocumentCommand\SecondOfTwo{m}{
  \exp_last_unbraced:Ne \use_ii:nn { #1 }
}
\ExplSyntaxOff
\DefTblrTemplate{caption-text}{first}{\FirstOfTwo{\InsertTblrText{caption}}}
\DefTblrTemplate{caption-text}{second}{\SecondOfTwo{\InsertTblrText{caption}}}
\DefTblrTemplate{firsthead}{default}{%
  \UseTblrTemplate{caption-tag}{default}%
  \UseTblrTemplate{caption-sep}{default}%
  \UseTblrTemplate{caption-text}{first}%
  \par
}
\DefTblrTemplate{middlehead,lasthead}{default}{%
  \UseTblrTemplate{caption-tag}{default}%
  \UseTblrTemplate{caption-sep}{default}%
  \UseTblrTemplate{caption-text}{second}
  \UseTblrTemplate{conthead-text}{default}%
  \par
}
\begin{document}
\begin{longtblr}[
    caption = {Some very informative caption.\newline I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text. I am writing some text}{Some very informative caption},
    entry = {Some very informative caption},
    label = {Table:1},
    note{a} = {In case you need a specific note.},
    remark{Note} = {In case you need a general note.},
    ]{
    colspec = {Q[si={table-format=2,table-number-alignment=left},c,co=.5]AAA[1.3]Q[si={table-format=1.2,table-number-alignment=right},c,co=.5]}, 
    width = \linewidth,
    rowhead = 2, rowfoot = 1,
    hspan=minimal,
    cell{1}{1}={l},
    cell{1}{Z}={r},
    column{1}={leftsep=0pt},
    column{Z}={rightsep=0pt},
    row{1-2}={guard},
    }
    \toprule
    K & b & t & pval & $\textrm{R}^2$~\
    \midrule
    \SetCell[c=5]{l} Panel A: Hi\
    \midrule
    10\TblrNote{a} & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    10 & 0.10 & 0.10 & 0.100 & 0.00 \ 
    10 & 0.10 & 0.10 & 0.100 & 0.00 \
    \bottomrule
\end{longtblr}
\end{document}