一般フォーラム

強引かつ見た目だけの解決方法なので，根本的な解決方法は有識者の方にお任せします。

以下の方法は，背景，枠線の色を白に指定した標準のboxの中にascoloroboxのboxを配置する，といったものです。

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\documentclass[luatex,fontsize=8pt,paper=b5,twoside,report]{jlreq}
\usepackage[top=16truemm,bottom=14truemm,left=12truemm,right=12truemm]{geometry}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%パッケージ関連%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\usepackage[most]{tcolorbox}
\tcbuselibrary{raster}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\def\ascb@textgt#1{\textgt{#1}}
              \def\ascb@gtfamily{\gtfamily}
              \def\ascb@zw#1#2{#1\zw}
\DeclareTColorBox{simple}{ o m O{.5} O{} }% 
{empty, left=2mm, right=2mm, top=-1mm, attach boxed title to top left={xshift=\ascb@zw{1.2}{11pt}}, boxed title style={empty,left=-2mm,right=-2mm}, colframe=black, coltitle=black, coltext=black, breakable, 
underlay unbroken={\draw[black,line width=#3pt](title.east) -- (title.east-|frame.east) -- (frame.south east) -- (frame.south west) -- (title.west-|frame.west) -- (title.west); },
underlay first={\draw[black,line width=#3pt](title.east) -- (title.east-|frame.east) -- (frame.south east) ;
\draw[black,line width=#3pt] (frame.south west) -- (title.west-|frame.west) -- (title.west); },
underlay middle={\draw[black,line width=#3pt](frame.north east) -- (frame.south east) ;
\draw[black,line width=#3pt](frame.south west) -- (frame.north west) ;},
underlay last={\draw[black,line width=#3pt](frame.north east) -- (frame.south east) -- (frame.south west) -- (frame.north west) ;},
fonttitle=\gtfamily, IfValueTF={#1}{title=【#2】〈#1〉}{title=【#2】},#4}

\begin{document}
\begin{tcbraster}[raster columns=3,raster equal height]
\begin{tcolorbox}[boxsep=0pt,colframe=white,colback=white,top=0mm,bottom=0mm,right=0mm,left=0mm]
  \begin{simple}{あいうえお}
    あいうえお
  \end{simple}
 \end{tcolorbox}
\begin{tcolorbox}[boxsep=0pt,colframe=white,colback=white,top=0mm,bottom=0mm,right=0mm,left=0mm]
  \begin{simple}{あいうえお}
    あいうえお
  \end{simple}
 \end{tcolorbox}
\end{tcbraster}
\end{document}

\begin{simple}...\end{simple} を \mbox や \fbox で囲んであげると なんとか格好がつくので、 どこかに改行 が紛れ込んでいるのかと試したところ 16 行目の最後の breakable, を除去するとお望みの形になるようです。

差分の提示も考えたのですが、一行が長いので見難いの上記の表現にしました。

なおページ跨りするような場合はうまくいかないかもしれません。

それとプリアンブルで @ を使用されていますが、 \makeatletter とかしなくてもいいのかな ...と思いつつも この点については深く検証していません。