Champions still able to auto though Riven's stun

Hello,

I've been spamming Riven top for the past couple days and a champion I know this bug works on is Gnar. We both had about 100 hp I opened with q and then e into the bush then I stunned him. While he was stunned he still got 1 auto off on me which caused me to die and unable to get my auto off. Let me remind you its before we were level 6 so he didn't have the attack speed to get it off that fast.

Thanks Burrito