"Essence creatures" are sometimes getting free from the ice

I've noticed, that when running into an essence dropping rare monster pack, their animation is not completely frozen and they are animated despite standing in one spot. But Sometimes if I move a bit further from them, they can "get free" from the ice the are in and start moving towards you and even "hit you once" and then their animation will stop again after that.

It happened to me yesterday, when I was in Act 2 "Traitor's passage" (spider like creatures with red dots).