Zalmarath The Colossus untargetable

I'm unable to progress as Zalmarath becomes completely untargetable whenever it moves to the right side of the arena. Nothing i do applies any damage whatsoever. And it doesn't seem to want to move away from the right side once it's there.

I've attached a drive video of the following bug. Crazy this hasn't been fixed yet

https://drive.google.com/file/d/1u4V8ZXYVVQgBmqiCO8eew97doQc779Zi/view?usp=sharing