Incorrect displacement when Tristana ult connects while Zac in flight (elastic slingshot)

While playing as zac I cast elastic slingshot to escape an engage. However Tristana cast her ult on me either before or during my flight. When I landed (a safe distance away) her ultimate's displacement took effect, however rather than knocking me back in the trajectory with which she cast the spell. It completely undid my elastic slingshots displacement vector, and then applied the trist knockback back from my original location. TO be clear, this was done as separate vector operations, first elastic slingshot was undone, and then I was knocked back.

This is not correct, by the virtues of multiplayer networking and latency, if I see the elastic slingshot animation and my character moving that is canon, it happened. It doesn't matter what tristana saw, even if she saw her ult hit my character model prior to launch, because my action resolved first.

And frankly it doesn't matter, there is a clear resolution in this scenario if both players observe a different resolution order (which they shouldn't because trist ult displacement should only be calculated on impact not launch [which obviously isn't how it's programmed right now]). SImply add the two displacement vectors rather than trying to undo canon (and physics).

I also fail to see how this should resolve differently than if I had been casting something more durastic like recall or tp. If I observed myself return to base/tp to my destination. I don't for a second believe any player would accept trist ult teleporting them back across the map so they can be knocked back by her displacement. Elastic slingshot should not resolve any differently. IE, as I said, her ult should have its displacement calculated at the point of impact and be an addition to any current movement vector not an override (it overrides the channeling, not a vector on the resolution stack).