It is a 12 year old legacy product. What specification exists other than, "Yesterday it did X when I clicked the button, but now it does not do that anymore."

This is the point: I don’t TDD, but i am a big fan of tests. I’m this case the incorrect spec can be flagged, but all the other incorrect specs will also be there. If your Fix doesn’t break a spec, great, but if it does you can check if that spec was correct. It’s a back and forth between code and business requirements