I believe the enemy wasn't stunned for the 1 second after fully channeling her ultimate. This happened a couple of times while playing w/bots.

This happened to me as well on Twisted Treeline when the enemy was Morgana. She caught all 3 of us and I was Olaf, Ragnarok'd it off. The other two did not but were likewise not stunned.

Can you give more details about the circumstances around the time you experienced the bug?
Do you remember which champions were involved?
Did it happen every time Morgana used her ulti? Was it only in one game or all the games you played were a Morgana was involved and the issue occured?

I've had this happen at least 3 times, it's the reason I stopped playing Morgana. Her ultimate doesn't fully channel sometimes. Not sure how to reproduce it, but I imagine this would be pretty bad if this happened during a tournament game.

This is a pretty common bug, I am really sad you still have not been able to fix it, it's been out for ages. Like 10% of Morgana ultimates don't go through, meaning that when enemies are still in the range they don't get stunned and applied the damage. Correct this fast now, that a Rioter got his eyes on it.

Finally a red that says something about it. This bug has been happening for months now, not just on the PBE, and it's completely unacceptable that it hasn't been fixed yet.

-It was vs. bots in lane and not in jungle.

-Can't remember which bots specifically but Leona and Warwick are ringing a bell.

-Classic Morgana skin.

-Haven't tested out again but I can try. Only that one game that I've noticed thus far but it looks like it's been an on-going thing. Happened at least twice from what I can remember.

I've had my ult break off even though they weren't off range.

Vayne tumble out of range so that was understandable.
Yet teemo went to a bush and was still in range and it just broke off.
Nunu was the only one that got stunned of the 3.