I reported a bug about this last night. I believe the issue is that someone raised the same time that everyone got the "Duty Complete" message (so they didn't get it because they were in loading screen) and since they didn't get it, the instance didn't kick them out (the game must have the "Duty Complete" message and the "kick everyone out" logic tied together.)

If anyone happens to see this, logging out and back in will kick you and your group out.