[luatex] luatex with plain tex

[Orders]

I run luatex for plain TeX using the miktex installation under linux
mint 21.  I upgraded the miktex installation today, and in-line formulas
no longer break at = or +, for example.  I am not sure whether this is a
problem with luatex or miktex, or something else.

A bit more background.  I use a modified (by me) version of weave for
handling modern Pascal dialects (Free Pascal).  I use a modified version
of webmac.tex to allow use of Unicode fonts through \input luaotfload.sty.

This has been working for many years now.

After today's upgrade I get a lot of overfull hbox messages.  As an
example

$\\{SOrganisation\_List}=\\{SOrganisation\_ABC}+\.{\',\'}+\\{SOrganisation\_DE}+%
\.{\',\'}+\\{SOrganisation\_FGHIJK}+\.{\',\'}+\\{SOrganisation\_LMNOPQR}$;\5

no longer automatically splits at the '+' signs.  There are many instances
like this.

I have tried changing \relpenalty and \binoppenalty to no avail.

Is there a change to luatex which might cause this behaviour?

Thank you.