[GAMEPLAY] Morgana's Ultimate Interrupted by Knock-Ups?

This bug I'm describing has happened to me twice so far with the enemy team definitely within safe range of the ultimate's radius. Playing Morgana, my understanding is that its uninterruptable, and that's why people can Zhonyas right after casting the ultimate and still get it off. But getting knocked up by Yasuo's tornado (maybe at the exact point in time where he was supposed to get rooted by the ultimate) caused it to cancel.