Einhar's Beastrite gem bugged

So i get the feeling that this lineage support gem is currently bugged. I have it on the skill cull the weak. I am culling rare monsters with it and gaining the 2 modifiers. However if i cull another (not rare)monster with the skill while the 40 seconds of the stolen modifiers are not over yet they just dissapear. Is this intended or is this a bug? And are we able to stack modifiers with this if we kill multiple rares in a few seconds or is that not intended to be a feature of this gem?

+1. If the buff is overwritten on next kill of rare monster than this is marginally better than Behead II and not really worth the gem slot considering the hoops you have to jump through to meet the buff conditions. If it is the case that buff is overwritten then the gem text fails to include that information. If GGG thinks this gem is too powerful, like Headhunter, and overwrite is desired in their design, would rather see a cap on the buff limit to maybe 10 or 20 modifiers.

+1 Same issue, and had me confused until i noticed it vanished when i would cull other normal mobs. I hope that is not intended, if it is the gem is absolutely useless.