> fixing it causes a breaking change. On my team whenever we come into this situation and realize that fixing overlooked things can actually "break" people, we really think twice about the fix.

Answered in the 2nd comment from Rasmus:

If this was changed in a minor version, I'd agree with you on the BC change, but we have been working on catching these weird edge-case scenarios that lead to unexpected bugs.

i.e. they thought about it and decided this version was an acceptable one for making breaking changes.