I also had the same issue tonight. Registered for Tam-Tara. When I commenced the duty, all 4 players were checked off, but when the instance loaded, I was the only one who hadn't been disconnected (presumably). So I exited and got the 15-minute lockout. Another issue, though, is that the lockout lasted well over 15 minutes. I was still locked out 40 minutes later with the duty finder UI stating that there were still 4 minutes left.