There are two things that resulted in not living up to your expectation.
- Kayle's a flying character. Her character's model, compared with projectiles that are usually low to the ground, can makes things appear like this.
- You shot Q from a position where the elevation is not equal compared to your intended target.
The weird thing that you should understand is, the animation (when a situation like different elevation occurs) does not match the actual hitbox.
There is no bugs in this video, just an unfortunate "side effect" of the game's functionality and design.