Bard stun bug?

So i was playing Bard , Bot lane, and 3 people tower dove me and i survived, thanks to the turret. But what happened is, the other champions dove me, and i was able to auto attack them and let the turret do the rest. I was able to hold off and slow the other enemies, who died to turret, and then stunned the Yasuo under turret with my q and the wall, and he died from turret shots, but it said executed. Both myself, and a few other teammates (also the Yasuo) commented on why i didn't get kill. I mean, i did stun him under turret, I'm not sure if I threw an auto attack or not, but he died right then and there. Shouldn't I have gotten the kill on Yasuo also?