[GAMEPLAY] Skirmisher's Devouer loosing as and not getting stacks from item

There's a problem with the Devouer.. I didn't really get the fact why does that happened but it happens every game with it. I main Yi and i was happy about bringing this sated one in the game. Okay,the bug is this:i farm the jungle (ofc) and try to get stacks(obviously) but when i buy the full item(to start stacking) while i farm the attack speed from the item dissapears as the wolf with it,the amount of stacks i had dissapear and the little wolf too. So,i realized that when you kill a jungle creep you don't get that stack while that happens.I've started recording it and got one good video with that happening on youtube.At first i was like:"Hmm,it might happen when i don't have smite active(is on cd)" but its not the case. So what is happening?