Smiting a champion with Challenging Smite doesn't give assist [GAMEPLAY]

Using challenging smite on an enemy just before they die does not give an assist. Happened 3/3 times in a game. Unsure if it (doesn't) happen with chilling smite as well.

Additional possible bug. Smiting a champion doesn't seem to be able to kill. Been dozens of instances where enemies has been between 10 - 50 HP and survived each smite. Not sure if it's a bug, intended, or freak healing incidents. EDIT: Talking about chilling smite here.