Proof:
http://www.warcraftmovies.com/movieview.php?id=235593
around 7:20.
Drain life/Mana isn't supposed to break when it's being channeled.
Right now it's completely useless, especially if combination with fear. A huge nerf that screws warlocks over.
It's been said before but so far I haven't actually seen any Dev confirm nor deny it. It's been reported on the tracker but nothing happens.