Honestly refactoring everything isn't an "improvement." More often than not it's a sign of that developer not understanding what the code does. Which is fine if they need to do that in a new code base or are otherwise rather green. Writing code is easier than reading it, after all. However they really should stash those changes or reopen a new branch after that exploratory phase.

IMO (in general) a good bug fix makes the smallest possible change to implement said fix. Otherwise the focus is about documention of the bug if needed and some minor refactoring to improve readability, consistency, or efficency