Tristana Rocket Jump not resetting

Always reproducible http://plays.tv/video/57ef2c1ba038bbf1b4/always-reproducible-tristana-bug

in this video, you'll see that the rocket jump never resets if you Buster Shot an enemy champion who is at 3 stacks of Explosive Charge, then hit them with an auto attack before the knockback finishes.

Trist E description: Whenever Tristana takes down an enemy champion or detonates Explosive Charge at maximum stacks on an enemy champion, Rocket Jump's cooldown is reset.