Redefinition of \genfrac is wrong

[eg9]

In mathstyle.sty we read

 145   │ \DeclareRobustCommand\genfrac[6]{%
 146   │   {#1\fracstyle
 147   │     {\begingroup #5\endgroup
 148   │       \csname @@\ifx\maxdimen#4\maxdimen over\else above\fi
 149   │         \if @#2@\else withdelims\fi\endcsname #2#3#4\relax
 150   │      #6}%
 151   │   }%
 152   │ }

but the second #4 is wrong.

See https://tex.stackexchange.com/q/456197/4427