Not sure if this is known or not. (I’d assume so with how old the game is)
But I talked to Vashek the stable hand in Uzhitz with 9 Horsemanship., and I was able to get advanced training (10 skill requirement). I believe this was because I have gold spurs on, which give +5 horsemanship., so I effectively had 14 as far as the game was concerned when I tried to train.
This got me actually to level 10 horsemanship. (15 if counting the spurs) I noticed the bug before I attempted to train master, because when I looked at my skill to see what level it was after training, I saw it was just now 10. That had me confused, since I should have needed to already be 10 to train, and then I gained a level.
So I tried master training with just 10 base horsemanship and +5 from spurs. … And it worked. I got to base level 11.95 (Almost 12).
Just figured I’d let you guys know.