Newly bred horses have empty hearts that used to heal, but now disappear

So, it used to be that you would breed horses, and the new horse would have a few empty hearts after you feed up the foal to adulthood using hay. These hearts would recover over time.  Recently, the hearts no longer heal, and if I come back to the horse later the hearts just disappear. These weren't unusually healthy horses, just in the 12-14 range. They end up with something like 8 hearts.