I agree with Disney horse except on one point. I agree that minis are probably no more difficult to GET in foal than larger horses. But I think we can't ignore the "elephant in the room" of dwarfism. I think minis are more likely to have an early abortion/slip the foal due to dwarfism, but it may not be noticed except as a mare that didn't get in foal that year.
My own experience is that you can have bad luck even with mares with the BEST foaling history, so I too, would be wary of this one if breeding is what you want.
ETA: Of course, if the stallion was running with both mares at the same time,. perhaps he just preferred the other mare! We hand breed, but our stallion has definite preferences and if Mira is in heat the same time as another mare, we have to hide her so Buckshot will focus on the other mare! Hide her somewhere and it's no problem at all....