Annie Stun Not Working With Tibbers

In a game I played as Annie it seemed whenever I ulted an enemy with Tibbers stun nothing happened. Tibbers was summoned on them and they were not stunned. I assume that perhaps I had just missed them or didn't actually hit the enemy champion? However, today a friend and I were playing and they said (I had not mentioned my issue with it) that they also thought that the stun was not registering when they used it with Tibbers. Has anyone else had this issue? Thanks!

Edit: I have not been able to replicate the bug in custom games. Perhaps it only happens if you have a certain item or something?

Edit 2: It seems that in both of our games we build a Morello and Rylais but we didn't have either completed when we first ulted. We just downloaded his replay so we're going to look at that.