Crowbell Map Boss Invulnerable

I encountered the Crowbell boss in a map yesterday and when immobilising it with freeze in the first phase (room) it did not proceed to the next room and instead became invulnerable when reaching a certain health percentage (around 50-75%) health. Nothing I could do would trigger it's next phase, and it would remain invulnerable but otherwise proceeded to attack me as normal. I was also unable to proceed or leave via portal. I tried the boss 3 times in a row but because I have cold attacks I froze him every time and could not defeat the boss and so had to forfeit the map.

A similar thing happened to me just now. He became invulnerable after coming out of a heavy stun. Seems the bug is triggered by effects that incapacitate it. Maybe it happens when he's supposed to go to the next phase while he can't act and the phase shift goes wrong.