XeLaTeX だけなぜか出力が違うソース

xetex-hyphenation.tex

% based on https://tex.stackexchange.com/questions/107208/
% XeLaTeX: NG
% LuaLaTeX/pdfLaTeX/pLaTeX/upLaTeX: OK
% plain *TeX: OK
\ifx\RequirePackage\undefined % plain
  \hsize=345pt %
\else
  \RequirePackage{latexbug}
  \documentclass{article}
  \begin{document}
\fi

\uchyph=0 %

\begingroup
  \lccode`C=`C
  Some filler text.
  Some filler text.
  Some filler text.
  Some filler text.
  Capitalised word. % Capi-talised
  \par
\endgroup

\begingroup
  \lccode`C=`c
  Some filler text. 
  Some filler text. 
  Some filler text. 
  Some filler text. 
  Capitalised word. % (no hyphenation)
  \par
\endgroup

\begingroup
  \uccode`C=`C
  Some filler text. 
  Some filler text. 
  Some filler text. 
  Some filler text. 
  Capitalised word. % (no hyphenation)
  \par
\endgroup

\begingroup
  \uccode`C=`c
  Some filler text. 
  Some filler text. 
  Some filler text. 
  Some filler text. 
  Capitalised word. % (no hyphenation)
  \par
\endgroup

\end{document}

Reported on latex3/latex2e#4

The "bug" can be fixed by this: https://gist.github.com/aminophen/44039ae0f78c63cca57781717ce13ec4