Varus' (Q) Piercing Arrow bug

When I play Varus, I don't like to use quick cast on the Piercing Arrow as I prefer to only tap the q button to aim and fire. Well sometimes the charge instantly goes away and shoots the arrow and it's really annoying when you're about to finish someone with a snipe and your shot shoots instantly for no reason and so you don't get the kill. Sorry I don't have a video or anything but my computer cannot record anything but this bug happens at least 5 times in every game I play as Varus.