[metapost] luamplib fails to process string with "char 0"

[Toby Thurston]

consider the following MP program, which is trying to print the Greek alphabet in the default font using the default TeX encoding, which puts Gamma at position zero

prologues := 3;
outputtemplate := "%j%c.eps";

beginfig(1);
string AB;

AB = ("AB" & char 0 & char 1 & "EZH" & char 2 & "IK" 
          & char 3 & "MNO" & char 4 & char 5 & "P"
          & char 6 & "T"   & char 7 & char 8 & "X"
          & char 9 & char 10);

draw AB infont "cmmi10" shifted 10 up;
draw AB infont "eurm10" shifted 10 down;
draw bbox currentpicture;
endfig;
end.


if I run this through `mpost`  (MetaPost 2.00 (TeX Live 2021) (kpathsea version 6.3.3)) I get two lines with the expected 24 Greek characters in each, with a box around them.

But if I change the program to wrap it up with luamplib like this:

\documentclass{standalone}
\usepackage{luamplib}
\begin{document}
\begin{mplibcode}
beginfig(1);
string AB;
AB = ("AB" & char 0 & char 1 & "EZH" & char 2 & "IK" 
          & char 3 & "MNO" & char 4 & char 5 & "P"
          & char 6 & "T"   & char 7 & char 8 & "X"
          & char 9 & char 10);
draw AB infont "cmmi10" shifted 10 up;
draw AB infont "eurm10" shifted 10 down;
draw bbox currentpicture;
endfig;
\end{mplibcode}
\end{document}

and run it through `lualatex` (This is LuaHBTeX, Version 1.13.0 (TeX Live 2021) then all I get are the first two characters in each string.  In other words it stops at char 0 (Gamma).
Oddly the bbox is still large enough for the whole string…

any thoughts?

Toby
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