Doing the helltide in Kyovashad and did the event to spawn the commander, Nemnom. Go to wail on him and he runs out of the encounter then runs back and resets to full health…constantly. You can kind of chill out on damaging him and he will start doing his abilites, but he just ends up one shotting you because if you start fighting him, he just wants to run off and reset.
You fail the mastery and you get the “event” with Nemnom just disappearing.
Playing a female Spirtborn running the crushing hand spiritborn guide on maxroll (in case it is an issue with my class/items and not the boss itself) on T2 difficulty.
Edit
If you kill him fast enough (I had the help of someone), he will not move to recreate the issue.