Yesterday I was playing as medic on pl_barnblitz when the engineers came into our spawn while we were still frozen. They started shooting at us but I guess we were not ubered because I started taking damage. Proof: https://steamcommunity.com/sharedfiles/filedetails/?id=2052846733 There are no engineers in the photo because they left. In the photo, you can see the medic in front of me also took damage. I am also pretty sure this has happened before.
Tested the bug twice, and each time no one took damage. I'm not sure how it would have happened the first time but I'll keep an eye out for it if it happens again.
Yeah, it's on any map and just happens. I've shot a medic after the game ended in their spawn and killed one.