Orianna's ult effect follows teleporting allies, but not walking or dashing allies. [GAMEPLAY]

Bjergson did the research for you. How it works (as you can see by watching the video) is that when Orianna has the ball on someone who's chasing an enemy, and hits R, the short "charge up" animation begins, but stays put where the ally was when you hit R. That means that at the end of the animation, the ult goes off in a different place than where the ball, and your teammate, is. This makes it more difficult to catch fleeing enemies when you have a teammate right on their tail and they should be in ult range.

It's an old bug, so I hope this hasn't been posted before. I did try searching for "'Orianna ult bug" but nothing relating to this specific bug came up.