Zalmarath act 2 Invulnerable to Damage

During boss fight if minions damage the boss he became completely immune to damage.
Minions cant damage him, my spells cant damage him, but he still can kill me.
Its like he falls of os his own arena

Same.

When it's working properly there's a short instance of invulnerability when it moves.
When it fails it's like the invulnerabilty is not removed at the end of the move. You then have to wait until the next move cycle and hope it gets removed then.
For me it was every second move it was invulnerable for a whole cycle, so roughly half the fight.

Only just started happening for me, was fine previously.