I was 1v1-ing a friend as Camille when he ulted right as I was going to jump on him. The ult was canceled and he returned to the correct spot. As far as I'm concerned, if ekko ults, he is supposed to go to the middle of the ring
when he ulted right as I was going to jump on him
I assume from what you said that your ult didn't land completely. Theres a brief delay when you ult as Camille between whet you press R and the activation of the ult (your time being invulnerable). If Ekko or any champ for that matter escapes before Camille lands her ult it's cancelled, but once you land it the enemy can't escape by any means, not even Ekko ult. His ult would be activated but instead of transporting him all the way back he'd land at the edge of Camilles ult. Here are some examples.
So you're suggesting that the ring should stop time?
I still think it's stupid that her ult can stop Olaf's Ragnarok
