Zalmarath the colossus immune to damage

When playing through the storyline (Act 2) with 2 of friends we all experienced Zalmarath the colossus being immune to damage.

We are unable to kill him now after several attempts because he becomes completely immune to all damage for 5+ minutes before we either die or log out.

I'm pretty certain that the issue occurs when the floor breaks, as each time we have tried this has been the case.

None of us have experienced this issue prior to the recent large update.

Please fix this issue as we literally cannot progress the storyline due to it.

literally just rand into this on cruel difficulty on my witch. emergency patch cause this stops progress for sure lol.

Same happens to me. This hitbox is so bugged

I tested with my friends and if he dies before the floor breaks then you can progress.

I have a high level character if you need a carry for the fight to progress?