Volibear Bite (W) not always halving cooldown when used on monsters

Occasionally, when using Volibear's W on jungle monsters, the cooldown is not halved as it should be.

I don't know what causes this bug, but it seems to be random when it just doesn't work. I've noticed it more when it doesn't kill the monster, but it also happens when it does kill. It's happened to me when chomping the big wolf near the beginning of the game and also later in the game on other camps.