When doesn't `\\` start a new line in `\caption`?

Question

Using the command in Why does parameter double when used in caption?, it works in most cases, but in a few cases (like 1 out of 20) \\ does not start a new line.

For example using

\begin{table}
\centering%
\begin{tabular}{llllll}
\hline
\textbf{Bezeichnung} & \textbf{x} & \textbf{y} & \textbf{z} &
$\mathbf{u'_N}$ & $\mathbf{v'_N}$ \\
\hline
... & ... & ... & ... & ... & ...
%...
\hline
\end{tabular}
\caption[Koordinaten ausgewählter Weißpunkte]%
{\label{\Lt{Weisspunkte}}Koordinaten ausgewählter Weißpunkte%
\FS[Quelle]{\cite[S.~279]{poynton_digital_2012}\index{Illuminant}}}
\end{table}

in a table does not start a new line before Quelle, like this (see yellow line; rest of page pixelated to emphasize the important parts and to improve image compression):

Or shown in detail:

Note that the same command inserts a line-break before "Bildquelle:" in the figure above the table. I could not find a reference to a similar problem; could it be that LaTeX tried to avoid the line-break if vertical space on the page is tight?

Additions

For those who failed to find the \\, here it is (answer from the reference):

\newcommand{\FS}[2][Bildquelle]{%
  \protect\ifthenelse{\equal{#1}{}}%
    {\protect\\{\protect\scriptsize{}#2}}
    {\protect\\{\protect\scriptsize{}#1: #2}}}

Forcing table 6.1 on a new page did not change the behavior. Making the caption text a bit longer or shorter also did not change the behavior. However when I changed the caption to

\caption[Koordinaten ausgewählter Weißpunkte]%
{\label{\Lt{Weisspunkte}}Lorem ipsum Koordinaten ausgewählter Weißpunkte%
\FS[Quelle]{\cite[S.~279]{poynton_digital_2012}\index{Illuminant}}}

then the table went to the next page, and the line break was inserted:

MWE

Finally I managed to create a MWE (pdflatex, actually it was easier than expected):

\documentclass[a4paper,twoside]{report}
\usepackage[german]{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{ifthen}
\usepackage[pdftex,colorlinks,hypertexnames=false]{hyperref}
\usepackage{makeidx}
\usepackage{showidx}
\newcommand{\FS}[2][Bildquelle]{%
\protect\ifthenelse{\equal{#1}{}}{%
\protect\\*{\protect\scriptsize{}#2}}{%
\protect\\*{\protect\scriptsize{}#1: #2}}}
\begin{document}
%
\begin{table}
\centering%
\begin{tabular}{llllll}
\hline
\textbf{Bezeichnung} & \textbf{x} & \textbf{y} & \textbf{z} &
$\mathbf{u'_N}$ & $\mathbf{v'_N}$ \\
\hline
A & B & C & D & E & F \\
A & B & C & D & E & F \\
A & B & C & D & E & F \\
A & B & C & D & E & F \\
\hline
\end{tabular}
\caption[Koordinaten ausgewählter Weißpunkte]%
{\label{Weisspunkte}Koordinaten ausgewählter Weißpunkte%
\FS[Quelle]{[Poy12, S.~279]\index{Illuminant}}}
\end{table}
%
\begin{table}
\centering%
\begin{tabular}{llllll}
\hline
\textbf{Bezeichnung} & \textbf{x} & \textbf{y} & \textbf{z} &
$\mathbf{u'_N}$ & $\mathbf{v'_N}$ \\
\hline
A & B & C & D & E & F \\
\hline
\end{tabular}
\caption[Koordinaten ausgewählter Weißpunkte]%
{\label{Weisspunkte2}Koordinaten ausgewählter Weißpunkte%
\FS[Quelle]{[Poy12, S.~279]\index{Illuminant}}}
\end{table}
\end{document}

Looks like:

Solution (https://tex.stackexchange.com/a/518731/182650)

After adding

\usepackage[font=small,labelfont={sf,bf},position=bottom,singlelinecheck=false]%
{caption}

the result for the MWE looks like this, which is OK for me.

Answer 1

You can simplify the example:

\documentclass[a4paper,twoside]{report}

\begin{document}
\begin{table}
\caption[]{aaaaaaaaaaa aaaaaa
ccccccccccccccc bbbbbbbbbbbbbbb \\ 
dddddddddddddd}
\end{table}
%
\begin{table}

\caption[]{aaaaa\\bbb}
\end{table}
\end{document}

Most classes set captions in two styles, first essentially in an \mbox on a single line that is centred if it fits in the text box (and \\ does not do anything in an mbox). If that box is too wide to fit on a line it is discarded, and the caption i re-set in a \parbox, If the second format is used \\ will work.

The caption package has an option to skip the one-line test, if that is what you want here.