A prime superscript in LaTeX/MathJax not displayed correctly

I have a (possible bad) habit of sometimes making a prime superscript in LaTeX with a 'hat' character in between, e.g., $x^'$ rather than $x'$. The LaTeX/MathJax rendering has worked fine either way for more than a year, right up to around yesterday, where I became aware that $x^'$ is not displayed correctly. See e.g. here.

1. What causes this problem to emerge now?

2. Was there a recent change to MathJax?

3. Is there a way to fix this problem without having to edit each and every post separately by hand?