Idk if it’s a bug, but it works just like in Expeditions
Unlike other monstrosities, Packmaster has a limited detection radius of 20m, so he won’t start attacking until someone gets in that range which means that yes sometimes you can fully avoid it if you know where he is
I think there’s some funny business with doors, sometimes he’ll warp like enemies do, then seemingly be stuck there according to radar mod and die. There’s actually a few doors in Expeditions that will despawn him if he enters (or worse, never let him out to roam in the first place).