I searched if any other forum posts about this existed that I can add to but didn't see any. The Moonhunter keep HM fight is bugged. Randomly the symbol aspect of the fight, only 3 safe zones show up, meaning one player will have to die. That makes the trifecta tough to do. All you can do is reset and go again and pray the bug doesn't happen.
Players tell me this bug has been there since the dungeon was released. I'd expect more from the company on bugs like these.