Just to tie into this - as it's a similar issue.

My bf and I were playing co-op, 2 player on PC last night, working our way through "The Order of the One Light" mission. He was on Drizzt, I was playing Bruenor. On Act 2, we had the final boss down to about 10% health, and had him sort of against a wall near where the room seals off, and the boss just... vanished. We'd cleared the adds, so we couldn't KO ourselves and reset the fight, so we had to replay the whole level (ugh!).

On Act 3, MANY of the cultist mage elites and mobs would just vanish shortly after being engaged/hit. If passing through the area again, they'll respawn and vanish again after being hit a few times. Super weird.

Definitely +1 what Josith says above about the Act 2 boss/lightning troll -- this is in the Companions of Icewind Dale mission, Act 2, final boss is called Stonegrinder (even though he has lightning powers, lol) That boss's AI is SOOO janky, like Josith said, he either stands completely still like he's frozen, but then suddenly within a few frames, gets a red outline, and blasts a major lighting attack before you can even react and dodge. Then he'll stand there like an idiot again (doesn't pursue at all).

This may be a personal gripe, and I know in D&D your dodges can fail, your attacks, CC's, etc. can all fail, so maybe it's meant to be this way, but maining as Bruenor, in general it feels impossible to tell what enemies can be knocked down/stunned/staggered. In one mission, I can literally chain knockdown a verbeeg elite, but in the next mission, knockdowns don't work at all on the same mob class. I wish it was a bit more consistent.

In general I wish all enemies would get a hard look at AI quality. Some enemies just stand and do nothing while you smash away at them. Others pursue you SO FAST that you can't get a moment's distance to pop a heal or potion. Biggest example of mobs like this are the Stygian chargers. When they spawn in groups of 3 or more, it's SO hard to get a break from their teleport/rush attacks.

And finally, from what I've seen, nearly ever mob type has no reaction to Cattie-Brie's ranged attacks (I'm getting this mostly from videos of her gameplay), she can just range mobs with zero counters or movement from them, and when overgeared, I've seen players run through levels with her just one-shotting nearly everything. I (think?) her damage is nerfed when she's at max range from mobs, but I think something else needs to change there.

This has happened to me as well offline and online. I'd be running or evading and also either get hit out of nowhere or run into an invisible enemy

Very active issue in PS5, co-op and solo

I had similar issues in solo both times I've tried to play offline. So it's not only multiplayer delta.squa#38860

Adding on to this, It appears to only happen in multiplayer, and doesn't affect offline gameplay.

(also PC, steam)