FNC vs SKT T1, Sejuani bug
I saw some comments on Youtube of some people who tried some recreating with that bug. Apparently, it's a purely visual bug which happens when you try to dash through some of the fatter walls in-game. If the wall is too fat, so your dash just about lacks the range to get past it, it will seem like you dashed through but you'll end up at your original location anyway. Again, purely visual bug. Not saying it shouldn't be fixed by Riot though, as it looks very deceiving in-game.