Forge Boss unbeatable after Patch with minions

Vastveld cant be damaged with minions after Patch cuz they dont Attack His arm.

Not only with minions. After getting him to transition to right side, as a monk I couldn't hit him anymore. There was no more hitbox so it became completely invulnerable. Tried to hit him with a Chaos Bolt wand and no luck as well. Ended up dying on his next transition to middle so couldn't confirm if at the middle I could have hit him again.

I either have to kill him before he moves to the right side or wait until he moves to the top which is super frustrating cuz the fight takes 4x longer than it should have and I am 4x more likely to die cuz of that.