Leona ult doesn't stun

Happened to me 2 times already that i played , hit the opponent with my R in the center so it shoud stun it and he didn't get stunned. I don't have a picture to prove it unfortunately, just cases I remember. Example: just now I played against DevilAngelXx who played as , I ulti'd him exactly in the center and he somehow used his W right after the hit. Immidiately after that, 2 people in the chat said it's a bug or a hack. It happened to me a second time against Fiora, who told me she already knows about this bug.

Pleae fcheck and fix it, I play Leona alot. ty