Invariant failed: Bad UTF-8 (full string verification)
There are other bad utf-8 assertion bugs (start of string, end of string). I think this is the first instance of this particular assertion failing so far in all our testing so far.
New tracking category
A good & detailed explanation of the new tracking category is at the English Wikipedia's page: https://en.wikipedia.org/wiki/Category:Pages_with_non-numeric_formatnum_arguments