Xerath stun does not appear to be working correctly [Gameplay]

There are three separate occasions that has led me to believe there is a bug with Xerath's stun. I will try my best to explain in clarity what happened.

1. I stunned a person about a milisecond before they flashed. This resulted in the person still flashing but they were stunned where they flashed to so I know the stun had hit them. I believe this was against a Veigar, but it was a few games ago.

2. I used Xerath's stun while Akali ulted. She would be stunned, but only after still dashing to me and dealing the damage from her ult

3. This one I am quite unsure about whether it is a bug or not. I stunned Rek'Sai while he was using tunnel. He was only stunned after emerging from the other side of his tunnel and not when the stun had actually hit him.