Warwick Blood scent bug?

So does anyone else find it odd that when an enemy goes invisible Warwick can't smell them with his blood scent anymore? Is this a bug or is this just a mistake on riots part? Warwick's Blood scent allows him to smell any opponent under a certain percentage of health, if your enemy goes invisible they haven't left area, so if they are still low enough to be caught by blood scent why cant he smell them?