Moodle doesn't load MathJax.js on our pages with MathML. It doesn't show up in either the HTML or a script-loader script call. I think the mathjax-loader filter isn't detecting the content. It works on our pages with TeX content.
We have tried the default MathJax.Hub.Config as well as several variations, most of which contained:
jax: ["input/MathML", "input/TeX", "output/CommonHTML"],
BTW, if we insert MathJax.js via Site Admin > Appearance > Additional HTML, the MathML displays correctly. So the issue appears to be in the filter. Is there some specific MathML string it's looking for?