This could be working as intended, not sure. If Nasus is chasing an enemy that runs around a corner or into a bush causing him to lose sight, when Wither is cast it will cause Nasus to stop running and perform the Wither animation. However, Wither doesn't get applied, seemingly from the enemy disappearing during the animation. This causes Nasus to lose multiple kills that he would normally be able to get. Is there something we can do to help alleviate this problem or is this working as intended?

This is normal, What the enemy did is similar (or essentially) to juking to purposely lose target rendering them to disengage the fight, which in results forcing them to run into the brush for a face check or waste a ward.