Lucian Ult and Stuns

Today I had an unusual experience. While playing Anivia, I stunned a Lucian while he was ulting. While the stun landed (the game clearly marks him as 'STUNNED'), his ult did not stop (thus I received the full force of it).

From what I could find, stuns are supposed to stop his ult. The only other thing I could possibly credit to this would be the timing. I've played it back a few times and it seems that he cast right as the stun lands. But even still, seems like such a weird interaction to allow the ult to continue when the game clearly knows he's marked as 'STUNNED'.

I may or may not take the time to get a clip uploaded somewhere and link here. I'm just curious if someone knows more about this, or has experienced similar things and could shed some light on the situation (otherwise I'll assume it really is a bug).