Twitch Auto Bug -- not full damage

Occasionally Twitch's auto will not do the full damage it was meant to and this really affects his laning. It especially hurts Twitch when he is trying to CS under turret. For example, after getting Serrated Dirk (maybe another long sword), you should be able to hit caster minions after one turret shot but occasionally, the minion does not die from it.

Here is an example of the auto not doing full damage. Deft puts one auto on each caster minion with 3 stacks of his passive. He Contaminates but one dies while the other one lives.

Edit: I think the interaction might be with the dirk passive. After Youmuus is completed, you don't notice it anymore.