Overbreeding Horses causes them to permanently lose health

On Realms I have a very large stable of breeding horses. I tag each horse with their tested attributes, including health, to make pairing easier. I have had two horses lose hearts compared to their tagged values. The first time I thought I had just mislablee the horse, but the second time it happened to my 15 heart horse: he lost 2 hearts permanently. I’ve bred him a lot, as he is one of my only horses with 15 hearts. Is this by design?