Yasuo wind wall blocking allied spells

I was playing a game as Sejuani. I was behind enemy lines when the enemy team dove my team 5v4 under tower. I came charging in from the back as my team retreated. I threw my ult. The ALLIED Yasuo threw his Wind Wall up to block incoming enemy skillshots. My ult hit his wall and vanished.

<--- (enemy team) | (Yasuo wall) <-------

I wasn't throwing it from behind Yasuo's wall. I was behind the enemy team and throwing it at them. It hit Yasuo's wall and vanished. It didn't explode, it didn't slow, it didn't stun, nothing. It just vanished, as it would if Yasuo was on the enemy team.

To recreate this bug, have Sejuani and Yasuo on the same team. Have them face each other at a distance where Yasuo's wall will come up between them. Then have Sejuani ult at Yasuo's wall.