We're using MathJax 3.x, which dropped support for inserting MathML into the DOM due to uneven browser support [0]. The example you linked uses MathJax 2.x.
Thanks for taking the time to investigate and explain the problem. I understand you're not at fault but the maintainers upstream, yet I can't help but be disappointed. Removing MathML doesn't help anyone, the user just has a worse experience; it's caused by the same brainrot as with Gnome.