Sion can pass through Azir's Ultimate with his own.
To put it shortly, if Sion activates his Ultimate a second time (canceling it early) as Azir's ultimate is cast (moving outwards, it hasn't stopped yet), he will end up on Azir's side of the wall.
Steps to reproduce:
- Play Azir vs Sion
- Reach level 6 on both characters
- Sion ults towards Azir
- Just as Sion reaches the space that Azir's wall moves (about 150 distance around himself, so pretty close), re-activate Sion's ultimate
- Azir can ult at any time while Sion is in this space, but Sion will still end up on Azir's side.
If Azir casts his Ultimate before Sion reaches him, the wall will stop him normally. This isn't gamebreaking, and it's pretty difficult to pull off, but I do not believe it's intended, so I thought I'd report it after confirming it with some testing /w the help of a friend.