[Gameplay] Vi E sometimes doesn't do damage in a cone.

I noticed while playing that sometimes my E cone doesn't show/do damage, but the minion (only thing ive noticed it so far) will take damage. I am using Demon and going Devourer and Tri Force and it only occured on a minion that I was dealing the killing blow on with my E. Still trying to get video, and also steps to reproduce.

Edit 1: anyone know of a website that auto records your games so i can get video?