Annie Ult stun proc bugged on Malzahar?

I recently played Annie vs Malzahar mid and Post 6 I would attempt to go in and ult Malzahar after I take his spell immune passive down, yet for some reason even though the damage from tibbers/ludens echo would proc immediately on malz it would never stun him. I used tibbers on probably every other champion in the game I was playing in and it seemed to work fine everytime. I just think someone should take a look in this as Annie's stun vs Malz is essential to win this lane.