Dark15719

This is a bug in the page, not the game, but I labeled it as Bedrock since I didn't see any other option to report this issue.

The graph for one just shows the stats of each generation. Below each generation (the distinction is not really visible in the image) the third parents' are shown, according to the old genetics mechanics.
The fact that negative health appears means that, at least according to old mechanics, the third parents are broken. These values are not measured, but calculated by taking the child's and parents' stats.

I made sure to get the correct health for each horse (my pickaxe helped me with the health).

Lastly, I get that I cannot assert this as being a true bug, but the fact is that it's not just a little bit more random, but in fact so much that it borders on not being dependent on the parents' stats.

I can't find many sources discussing the mechanics of horse breeding, and most are pretty old, so they might have changed through the versions.

But aside from that, what I'm saying is that the health of the foals seems much more random than any other stat they have.
As an example: of the 12 children from a 14 and a 15 hearts health horse, only two had 14 hearts and one had 15 hearts, while the lowest was 7 hearts.

I am not sure if this issue would occur in a local copy of the world, as it is difficult to test and takes quite some time.
The issue does not relate in any way to the bug-report that you shared, although that bug has also occurred to me.

By a "third parent" I refer to the mechanic that is used to insert randomness in the generation of offspring stats when pairing horses. As far as I know, the stats (or genetics) of a child are determined by taking the average of the parents' data and a random "ghost horse", as it is sometimes referred to. This "third parent" does not exist, but is a randomly generated set of horse stats, generated for each foal.

After some further breeding, the health indication bug is becoming a real problem. Before, I could just reset it by restarting the game.
But now, it won't go away, no matter what I do.
Some older horses still retain their health indication, but many newer ones only show up to 9 hearts.

Full hearts

After being hit once with a diamond pickaxe

After the second hit

The hearts do not reset after restarting the game. I tested the value, by trying to heal it, and it remained constant and lower than I had seen before accidentally hitting the horse.
I constantly see visual health bugs, but this was not one.

It seems as though the horse took its previous health and not the health stat to determine its new maximum health after being tamed

On a side note. I just got another horse with a bugged out max health. Its max health are 7,5 hearts, but the health bar displays a maximum of 9 hearts (with 1,5 empty ones). I tried healing it with wheat and apples but none worked, as if it already had a full health bar.