Friendly Sion Ult stops when hitting front side of Azir ult

So to be more specific, my friend playing Sion, ulted my lane, (I'm Azir) I do a shuffle on my opponent, they flash thorough, but Sion is right behind me. Next thing that happens is Sion hits my wall, and instead of going through it, just stops on it. IT DOES NOT CANCEL HIS ULT, he just pauses on it, then when the wall is gone, he keeps going, still in ult.