Azir ult bug

Hi ! I was wondering if it was a bug or not ... but can Azir ult stop stop the sion ult (unstoppable onslaught) ? Because it just happened to me in a game and the sion ult description is that its not affected by crowd control ! So theoretically Azir ult should not stop the sion ult. Anyway thought you might want to know if it is actually a bug.