Sion Leap does not activate Sudden Impact.

An official tooltip (Did you know) that displays "Sion can hop over small walls with the final leap of his ultimate." Sudden Impact does not activate on this leap. There is a very simple way to fix this since Sion's ult is considered a channel. If the player recasts or reaches the end of the duration of the ult, grant the buff. Those are the only ways Sion can Leap.