Just cheking if this is a bug

So i was playing Vayne normally but i couldn't farm... it was because that my target doesn't take dmg until the arrow hits him completly. And in teamfight i attacked Kog'Mow but died before the arrow hit him (the arrow was in air when i died ) but nothing happened cause he got 2 stacks of W and 100 hp left so the last hit would kill him but it didn't. And this happened again in another fight.. I don't know if this is a vayne thing or a bug so please tell me and ty.