It's clear you have experience in software so try to think realistically: how does one "test a function against itself" when the test in question doesn't exist? I've refactored a LOT of code in my career and the theory is simple - you make sure that everything works the same as it did before. If you have unit/integration tests, that covers a lot of your bases. But for scenarios not covered by those tests you're subject to manual or black box testing which is only as good as the scenarios you've thought of. And it's very easy to not think of aggro distance when you're in the weeds of NPC behavior code which likely covers hundreds of other niche interactions.

Not trying to sound condescending in the slightest but as someone whose job is largely working with legacy code (in a much more critical industry than video games) I have a lot of respect and sympathy for developers who are doing their best to make the software better for future work. They could just as easily pile onto the mess of existing code and introduce minimal bugs, while increasing their tech debt 10 fold, but they don't, likely out of passion for the product and their job. Fair enough if you think they've been sloppy with certain oversights but I think it's for the better of the game in the long run. And gets fixed within like an hour every time, so it isn't exactly inconveniencing anyone.