[ART] Vayne Crit Hit Glitch?

So during my recent playing as Vayne, I started seeing spears of light coming down from the sky on targets I was hitting; I notice it most on hitting minions, I can't tell if it appears on champions (since I'm focusing my three-hit W proc). This spear kinda looks like a narrower, faster, silver-white version of a Karthus ult.

The weird thing is, the spear doesn't appear on the third hit, nor does it appear (consistent) on unit death, and sometimes procs while the target is alive. I realized later that this spear consistently appears on critical hits; I notice the spear only appears after I purchase Zeal/PD/CoA/IE and appears more often when I have PD and/or IE.

This isn't a gamebreaking bug obviously but it is sometimes distracting, and sometimes irritating, like looking at a faulty strobe light.

Has anyone else besides my girlfriend (also a Vayne player) and I experienced this?