Here is a screenshot from my activity page:
running Google Chrome Version 99.0.4844.51 (Official Build) (64-bit)
However the browser or the version doesn't seem to matter, tested same thing on Firefox, Waterfox, Edge (running stable main line).
It is pretty evident that the MathJax is leaking out of the box (when it shouldn't). I found a similar question which has now been marked status-completed but looks like this question deals specifically with multiline formatting, so I think it is not the same bug.