Zalmarath the Colossus doesn't take damage

My friend and I were fighting Zalmarath the Colossus and he would just randomly not take any damage from either of us at some point during the fight. We tried it 4-5 times and each time at different points during the fight he would just stop taking damage. Also, the 1 hit kill knife hand he does that you can't really judge where he's going with it is ridiculous.

I also have been having the same issue. At some points I can't target him anymore and just have to dodge attacks until he moves. Even when he moves tho it's not guaranteed to let me do damage. I hope this gets patched soon

Been having the same bug since December, quit the game for a while because my progression is soft-locked. redownloaded this morning and the bug is still there, hasn't changed whatsoever. After taking a certain amount of damage, Zalmarath becomes invulnerable to all of my attacks until he switches positions, but sometimes he never switches positions again, soft-locking the fight. I thought maybe it was a problem specific to my witch/ranged attacks but haven't been able to pin it down. I'm reluctant to continue playing the story on another character because I haven't been able to get enough information about this bug. PoE2 is my first time playing PoE, never played Diablo, and I was really enjoying the game before this bug. Sad stuff.