[luatex] Upgraded luatex sub/super scripts

[Orders]

I run luatex for plain TeX using the miktex installation under linux
mint 21.  I upgraded the miktex installation today, and single-character
subscripts and superscripts cause an error.


For example, $x_1$ produces the following error

! \textfont-1 is undefined (character 49).
l.2 The incorrect variable is $x_1$

Enclosing the sub/superscript in braces ($x_{1}$) fixes the problem.

A minimal test program is as follows:

\input luaotfload.sty
The incorrect variable is $x_1$.  The correct variable is $x_{1}$.
\end

Is there any change which might cause this?  Is this a miktex problem?

Thank you.