Boss fight - Steaming Springs map with delirium modifier

I was doing a Steaming Springs map and added a Delirium modifier to the map. Halfway through the fight, the boss disappeared and only the Delirium apparition that was appearing every few seconds remained, but the boss didn’t show up again. I couldn’t leave the arena or teleport out. I had to select "Exit to Character Selection" and come back into the game. I tried to go back to the map to the boss, but it still didn’t appear. I couldn’t complete the map because of that.