This was happening to me at the weekend, but didn't happen at all yesterday, think they're fixing it. I found another bug, I have the free Arabian, worked him up for races and set him as default for racing. But I happened to be (role)playing around on my big Shire horse when I entered Race Series and turned up on the Shire! I looked such a fool on the big mare, at a race!
Still won it though, which was even more satisfying, so I think I'll take her racing more often. It feels great punching someone out from up there, makes me feel safer, though she takes turns like a tank...