I remember when wraithlords helmet was added that the minion from it was drifting like crazy and that it was fixed if I recall well. Now the bear is going crazy and starts drifting quite often. The clip in the link is what happened after its drift ended. He went on like that for a whole minute while I was staring at it and it only stopped once I got the nemesis to move away a bit. First time it happened though to not attack the target next to it.
Also to add to this idea of monsters being weird I have seen some of the t-rex enemies charge through walls or places where there is no ground while in the new area where you have to kill the rift beast and hatch its eggs.