You should have recorded the few seconds before ulting as well. It would have been easier to diagnose.
No matter where your cursor is, where Sion is facing is where he will begin his ult. Doing it immediately after teleporting is a big gamble because of how the character is situated when teleporting. They teleport literally right on top of the minion, and I think there's a code that makes it so they will be on the side closest to your base (it works this way with turrets), so the first split second after teleporting can be kind of silly sometimes. Turrets are a big thing, though, and wards have no collision, so there's no problem with them. Minions, however, are tiny and constantly move, and that becomes a problem when just trying to move normally.
It's not a bug so much as it is just bad collision with units. You can't just "hide" a bug. That requires more effort than it is to just fix the damn thing.