Gameplay - Leona's ult doesn't cancel some abilities when it stuns.

I have been playing Leona a lot recently and I have noticed that some of the times she ults and stuns someone who's in the middle of casting an ability, the target will not be able to move and will have the stunned icon but the ability will still cast. 2 examples of this is when Aatrox uses his jump right as he is stunned, he will jump to where he was supposed to jump and then be stunned where he lands. The other was when Sion uses his q, you can stun him while he is channeling and it will show that he is stunned, however his q still channels and follows through with the ability like he is just slowed instead. I think its a bug where the game recognizes the ult as a slow, only immobilizing units instead of also cancelling what they're doing.