Breeding horses gives empty health hearts that can’t be filled

Horse health starts having empty hearts after a few times of breeding. For example, if I breed some horses that both have 14 health, the baby comes out with 13 hearts but only 9 are filled. The other 4 hearts stay empty as if it needs healing, but you can’t heal it, and the horse is affected as if it only has 9 hearts. (The picture I attached is just showing the empty hearts)