Yasuo or Lissandra bug

I just played a game where first blood should've been mine but the game decided NOPE and made the kill an execution.

I was playing Lissandra and my opponent was playing Yasuo in the mid lane. He tried a really ballsy turret dive and as he went in, I stunned him under tower and Q'd him. The tower killed him but even though I hit the Yasuo with my W, Q, and my E, the game counted it as an execution. This then caused me to tilt as he drew first blood with Vi's gank. I fed 8 deaths to him and he carried his team to victory.

Please fix this ;-;