I am not sure about this interaction and whether it is a bug or not.
I had a Voracious Arachnid on board which had Cripple Blow and I also had Rankett: Proving Grounds. I killed an opposing ally and I was allowed to activate Rankett's ability although no ally got actually buffed this turn. The only explanation is that Arachnid's buff, although invisible with CB it still counts as base attack buff for the location. Is this the intended behaviour?

Game reference: BP Holy Punisher (Moonstalker) vs EBHYX (Vess) at Turn 8
Game version: 3.501