Mixed breed?

[Emiisheartless]

Hello everyone! I've been breeding horses a bit randomly lately and noticed something strange. I started out with a turkmene and a half belgian, half forest horse. The resulting foals were, as expected, half turkmenes. However, the next generation threw me off a bit. I bred the siblings together, but the foal ended up being 60% turkmene, instead of half like both its parents. Continuing to breed full siblings or cousins, I recently got a foal that was full turkmene, despite only half the horses in the 6th generation being turkmenes. So, my question is: how do part breeds actually work? Is it somehow cumulative or is this some sort of bug?
I'll link the horse in case anyone wants to check it out.
Weird breed?