Value of LuaTeX’s \mathstyle in \frac (de)nominator

\frac{a}{b} is essentially {a \over b} and the big problem with the infix \over is that it makes tracking the current style hard luatex does not change that.

to get a version in which \mathstyle is set you need \Ustack{a \over b} so in latex you would have to redefine \frac and related commands to use \Ustack.

One possibility would be to load lualatex-math which redefines \frac in this way.