Riven ult unable to cast second time. [GAMEPLAY]

I just got out of a game as riven and had a few occasions where I ulted and couldn't recast for the execute. I was fighting a leona top, and to begin with I assumed I was stunned and couldn't get it off in time. But it happened multiple times, and each time I seen the cooldown timer start for it before I actually died or recasted the second time. I am not sure how to recreate the bug as it was not happening all game. It only seemed to happen while I was fighting leona (which was most the game but I roamed a few times) and it was not 100% of the time I fought leona either. I also had another bug where she dove me and I got stuck at turret for about a second bobbing up and down.

I have the baronreplays file but cannot upload it, I will try uploading to an external site and linking it in a bit.