It's an odd thing. Just take rejuvenation, set it's Hit Points Gained to -30, and allow enemies to be targeted. You'll see that once the Duration expires the life drain is not removed.
Edit: Further testing reveals that if you use a normal unit to cast it works fine, but if you use a dummy unit this problem occurs.
Edit 2: I feel like this is one of those Blizzard hard coded issues. Is there an alternative you can think of to achieve the same effect?