Not sure if the recent patch screwed this boss over or if this has been reoccuring. After the second phase, killing the big skeleton dude with the three Dreadhorns, the three dreadhorns will just outright not respawn. Had this happen 3 times. The first time we tried porting out and back in and nothing happened. The boss was just completely gone and would not reset. After that we tried to make a new group and just bite the bullet and redo the dungeon. Got the the third boss again and it did the exact same thing. We did nothing special or game breaking to make this happen, the three bosses just refuse to revive after the first time.