I was in a game when i was coming from mid to gank. As I Q'd in the enemy nocturne used his "fear chains" (can't remember the skill name) As he did, I ult'd before they activated. It didn't cut the leash because I didn't move out of range, but Noct's skill activated as I was slamming him to the ground and it didn't damage him.

I don't think that's a bug, that's just someone cancelling your ult/negating it which I'd think is something they added on purpose.

but being knocked up is a stun but it might just be luck of the draw.