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.