So at Shear Point, there is a coffin that busts open and Krosis pops out. This guy, I could not withstand his fireballs so I had to dodge each one and slowly whittle him down with weak spells from dodge distance. But at some point he just stopped hurling spells and just flew around, ruining the danger. This is normal dragon priest behavior? The only mods that could be affecting him are ASIS + Apocalypse Spells. Behavior AI on ASIS Skyproc is unchecked.

Sometimes they bug a little when they use a lot of magicka reserves/prolonged battles. Staggering them(unrelenting force) usually snaps them out of it.

I've had an issue with Morokei doing the same thing but I assumed it was a quest related bug. Thats because when Estormo (the Thalmor mage) who meets you at the end attacked me he immediately stopped attacking me after casting a few spells. I decided to keep him alive (unlike Morokei of course) and he just followed me around. So I basically had a third follower for about 10 levels (he was a damn good follower) until he died during a dungeon dive.

I had a similar pacifist bug when fighting Harkon.