Horse Won't Heal

I bred this horse that can jump 5.5 blocks and run at 14.23 blocks per second, with a maximum of 14 hearts. However, it never heals beyond 13/14 hearts, no matter what I feed it. I have tried feeding it golden apples, hay bales, and wheat, but its health always stays capped at 13 hearts. I’ve attached a video showing the issue.