I have confirmed that, in the case of dvipdfmx,
\documentclass[dvipdfm]{article}
is ok, but
\documentclass[dvipdfmx]{article}
is ng.
Therefore the bug may be in dvipdfmx.def and xetex.def.
However,
latex --> dvi --> dvipdfmx --> pdf
for
\documentclass[dvipdfmx]{article}
\def\pgfsysdriver{pgfsys-dvipdfm.def}
\usepackage{lipsum}
\usepackage{tcolorbox}
\tcbuselibrary{breakable}
\begin{document}
%
\rule{0.4pt}{17cm}
%
\begin{tcolorbox}[breakable]
\lipsum[1]
\end{tcolorbox}
\lipsum
\end{document}
and xelatex for
\documentclass{article}
\def\pgfsysdriver{pgfsys-dvipdfm.def}
\usepackage{lipsum}
\usepackage{tcolorbox}
\tcbuselibrary{breakable}
\begin{document}
%
\rule{0.4pt}{17cm}
%
\begin{tcolorbox}[breakable]
\lipsum[1]
\end{tcolorbox}
\lipsum
\end{document}
generate good results.
Thus problems are in pgfsys-dvipdfmx.def
and pgfsys-xetex.def.
Best,
Akira
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