6Seaweed9

Related bug reports:
MCPE-185089
MCPE-223582

Additionally, the bug is still present in a newly generated creative world.
Steps to recreate:
generate creative world.
spawn two horses with spawn eggs.
breed the horses.
tame and saddle the offspring.
ride the offspring to a magma block and watch its health react unexpectedly.

Update: The issue still exists even when breeding horses generated in a newly loaded chunk. It does not require specific or “old” horses. And can appear in the first generation of offspring. It does not require multiple generations of offspring.

The Issue is still present as of 1.21.111
The Issue seems to stem from a discrepancy between displayed health values and actual/apparent health. And only seems to affect bred horses.

As pointed out by the commenter from 11/Sep/24, the discrepancy can go in both directions. Either extra hearts are displayed that are always black “unfilled“, OR additional apparent hearts are not displayed.

In the video above there is an adult bred horse that will consistently take 6 damage points for it’s visual health to be decremented. The bug seems to be purely visual, and the actual health seems to be consistent. The bug can be briefly “fixed“ by unloading and then loading the chunk that the horse is in. The bug reappears after the horse takes damage. As shown in the second video with the same horse.