Zalmarath issues

My group and I have also had the invulnerability issue. We find it happens when a melee character gets too close to him. Zalmarath either pulls you in to his hitbox off the map or in our case, my ice walls pushed our warrior into him and then he becomes invulnerable again. It was pretty consistent when I was putting my ice walls on him and the warrior was in just the wrong position.

Worth looking into as it is pretty easy to replicate.