Fiora interaction with Udyr bear stance

Unfortunately I don't have any video evidence of this but I just recently discovered what I hope is a bug and not a feature.

When I was playing Fiora recently I ended up being ganked a few times by the enemy Udyr, he tried using bear stance to stun me but I used Riposte on his attempts. However I noticed that if the riposte follow up hit him it didn't cause a stun like it normally should, I'm not quite sure why it happened but I feel like this needs to be addressed seeing as it's pretty game changing depending on your perspective.