Morgana Ult Not Stunning After Full Duration

Several times tonight I have kept targets tethered for the entire Morgana ult and they would not get stunned. I couldn't see if they took damage or not so I'm reluctant to put that in, as well, but if someone can explain to me that if whatever is causing the no-stun bug to happen also cancels damage, that'd be very insightful.

However! This is not EVERY ult. The inconsistency is aggravating.