I wish I was recording to capture it, and I wasn't able to recreate it to try and confirm what caused it. Somehow our fireteam managed to get Rhulk to spawn inside the structure that floats above his final fight arena and it prevented him from using the laser mechanic. We resolved it by wiping as a fireteam and starting over. I have a theory that it was because he was too close to the stairway to his arena when he went immune and it made his positioning weird. Just wanted to bring it to light if it hasn't been already! Good luck out there.