Essence Monster Bug

Had an Essence in an Abyss map. Small untargetable and unkillable spiders spawned nearby, before activating the essence. Followed me through the entire map. Seems like a standard mob for this map as there were more of them all over, most of which were easily killable, but there were always 1-3 unkillables following wherever I went. (Fortunately low damage, but very annoying.)

One even sat twitching in the arena for the boss and immediately jumped me after. I then noticed a rare in a cleared area and went back to check. Nothing there so I went to finally trigger the Essence - which was in a different location. Turned out the "invisible" rare nowhere near the Essence was the Essence monster (didn't notice which one as minions killed him too fast) and, after killing him, the annoying unkillable spiders went away.

Just a little pain in this case. Would result in a lost map if the unkillables were more dangerous.