文字コード256の \chardef が化ける

chardef-math-256.tex

%#!uptex -ini
\catcode`\{=1
\catcode`\}=2
\catcode`\$=3
\catcode`\^=7

\font\FC=ec-lmtt10 at 5pt
\font\FB=ec-lmtt10 at 7pt
\font\FA=ec-lmtt10
\textfont0=\FA \scriptfont0=\FB \scriptscriptfont0=\FC

\font\IC=ec-lmtti10 at 5pt
\font\IB=ec-lmtti10 at 7pt
\font\IA=ec-lmtti10
\textfont1=\IA \scriptfont1=\IB \scriptscriptfont1=\IC

\font\SC=cmsy5
\font\SB=cmsy7
\font\SA=cmsy10 % math symbols
\textfont2=\SA \scriptfont2=\SB \scriptscriptfont2=\SC

\font\EA=cmex10 % math extension
\textfont3=\EA \scriptfont3=\EA \scriptscriptfont3=\EA

\font\JC=upjisr-h at 15pt \JC
\font\JB=upjisr-h at 17pt \JB
\font\JA=upjisr-h at 20pt \JA
\textfont4=\JA \scriptfont4=\JB \scriptscriptfont4=\JC

\fam0    \FA
\jfam4   \JA

\hsize=300pt \vbox{%
\chardef\AZ=255\relax
\chardef\AX=256\relax
\chardef\AY=257\relax
$[\AZ A^\AZ]$, $[\AX A^\AX]$, $[\AY A^\AY]$}

\hsize=300pt \vbox{%
\kchardef\KZ=255\relax
\kchardef\KX=256\relax
\kchardef\KY=257\relax
$[\KZ A^\KZ]$, $[\KX A^\KX]$, $[\KY A^\KY]$}

\end

See texjporg/tex-jp-build#129