When fighting nurgloth the eternal during the Halloween event I and my team discovered that about 3/4ths through the fight his invincible status had become permanent. Just before his 3rd phase where he starts swinging at you with his scythe.
Our comp was: Waystalker, Slayer, Bounty Hunter, and Pyromancer.
The status he has when he is up on his perch suddenly became permanent and we couldn’t damage him.
I am unsure as to what caused it, but it had never happened to me before even though I’ve beaten enchanters lair a few dozen times. I will update if this happens again but I wanted to make it known now as it was incredibly annoying.