Why didn't it hit?

That seemed a bit off, so I went to practice tool n tested. The cast range indicator and visual animation for Rivens R are not that accurate, but what I found is the biggest visual discrepancy lies with that skin. For her Default ulti, she has Green bladed things that fly out, at the end of its cast range the blades dissipate and then its just green energy that extends forward a bit but it wont do damage. The projectile thats the bladed parts of that skin (and arcade for that matter) Do not dissipate and extend to the end of the animation. This visual does not match expectations for its hitbox, Its a bit more clear on her Base skin and others that share that R as the blades melt away but championships do not have that in their animation.

If you look closely, the trails that each blade is leaving became less thick and began to rapidly disappear right before reaching Zed. That's when those projectiles stop mattering