Maybe this is a bug, but the key you get doesn't open any doors that I could find and definitly not the door you originally opened.

Catch me in game sometime with this character to try to figure out what went wrong.

(only thing I can think of is somehow a DM or PC locked a door on their own)

Well, I can check after a reset for sure.

News on this?

Yes, PCs can apparently lock that door. Then you can't get through.