Happened to me too. But apparently it was because I was invited to the group finder after someone had left, so technically it was overuling the original people chosen by the finder, but I don't know for sure. It also wasn't showing my role next to my name. I haven't tried it again because every time I try to do a random dungeon I never get picked because I'm not a tank or healer, but a damage dealer. This was on PC.