So... This isn't the bug you think it is. The same behavior can and does show up on Mathematics, and they've lamented it for years:
The insidious bit of this is that sometimes, it seems to work - the diff engine and MathJax processor interact in juuust the right way to both render the expression properly and highlight the change...
But usually it doesn't work, and you're lucky if you only get unrendered MathJax; if you're unlucky, you get a hash of partially-rendered expressions making the diff all but impossible to decipher.
A proper solution here is probably to treat MathJax expressions as atomic, in the same way we do for (say) images: if there's a change to any part, the entire expression is marked as added/removed. This would highlight the change, albeit not with any real granularity; you'd have to carefully examine complex expressions to determine what was different. Making this happen would require some amount of awareness of Mathjax within our diff engine though, (and/or some awareness of our diff engine within our Mathjax postprocessor) so it's a non-trivial change.
The work-arounds for now are as follows:
Switch to "markdown" diffs. This shows you the raw expression, complete with granular diffs.
Hit "Improve" and view the preview. No diffs, but at least the expressions are rendered properly.