It was pointed out earlier today on chat, by Martin Sleziak, that the scope of MathJax new commands changed at some point in the (recent?) past: it was switched for Math SE in January of last year and on this site at some point in the intervening months.
The short of it is that answers now start and end with
markup, which scopes any
\renewcommand instances to only that post and not the entire thread.
This is Good Thing, and indeed something we've complained about before. However, it does also mean that answers that used commands defined in the question may now be broken, and it'd be good to have a thorough search for them and implement any fixes that may be required.
As such, I'm using a SEDE query, Answered questions with LaTeX \newcommand's , which lists all the relevant questions, in case anyone wants to help me look through them.