The exact same thing happened to my group. Firestorm didn’t reset to inside the arena, we pulled him off of the bridge (because that’s where he was standing) and “killed” him there…he went invulnerable and got back up.
After 4-5 more deaths, the dungeon leader left and we were all so very disheartened.
Also, Agony on entering the arena? Not functional if the party is supposed to survive.