I had this happen twice now. The first time was when I was playing Azir and second time was when I was playing against him. Both were in ARAM. In both case there was no spell-shield or other ability that prevent cc.
It's generally because you ult before your E knock-up lands which messes with the game a bit so it doesn't apply the knockback, if this was fixed however Alistar would become trash-tier, a rioter has said it would be possible just to rewrite Azir so this only applies to Azir but this seems to have gotten a little bit pf a negative reaction from the reddit community.
