It doesn't look like a bug to me. The key here, I think, is her pick rate. The fewer the number of players she has the more likely it is those players know how to play her well.
I had a win rate of ~70% on Garen before the juggernaut update, so if it's really only Kayle mains playing her right now this could explain it.
Just keep in mind sample size does matter. I ended season 3 with a 100% win rate on Taric. It would be an impressive feat if you didn't pay attention to how many games I played of him, which was only one.