[GAMEPLAY] Swain's Torment not registering

Whilst playing Swain top lane, I have noticed what I believe is a bug with swain's E (torment). Whilst casting E on an enemy chap if they step into the bush before the animation hits them than the effect never takes place on them, this wouldn't be a problem however it also than places the spell on cool down for you, meaning either it hit them and hasn't registered or it hasn't hit them and I should still be able to cast it. I've tried this over multiple games and found that it happens regardless of the bush they walk into. The only exceptions I have found is when you cast it over a wall it will register even if you loose sight of the champion you've cast it at.