When you get hit by serrated knives the debuff does not show on screen. It just deals damage to your character but does not show how long the duration last or how much damage it deals.
In addition, when you are Shiv it does not show stacks of the Serrated Knives under (or is it over?) the enemy HP bar. It is as if Serrated Knives is not a "true" stackable debuff, although I see no reason why it shouldn't be.