Rathbreaker

I got Rathbreaker to half health and then ran to the middle of the arena down the cliff to kite the minions and Rathbreaker went back on top of the cliff and just sits there. I can attack him and continuously damage him, but he just stands still and regens health making it impossible to kill.

yes, i confirm this as well, here's a video of him just acting high and mighty and collecting dmg
https://streamable.com/4n9zfy

I can confirm I have run into the same behavior, by running down and back up.

I ran into this bug not too long ago, I retreated down from the upper area into the hyena pit and it displayed the same behavior, no long interacting and infinitely regenerating.
I 'fixed' it by dying and respawning at the checkpoint to reset the fight/area, I imagine this is not a desirable fix for Hardcore characters.