Zalmarath Immortality (Cruel act 2)

Pretty self-explanatory, I went in to his arena, he started taking damage then abruptly stopped taking any damage. He was like this for a minute before I decided to die to reset him.

I believe this may be related to him moving, but it's hard to say for sure. It probably happens on both but this one was in cruel.
https://youtu.be/SwSLVcmzkCs