Kalista E bug [No CD reset?!]

During a bots match I played I noticed that when I was utilizing the mana regain (Killing at least two units via Rend (E)) that Rend (E) would not have it's CD reset, but it would still provide the mana. Not sure if this is intentional, or if the Rend is just bugging out.

Will post a video of the error later.