The bug

Unlike with the player's health bar, the horse's health bar's background does not flash white either when taking damage or being healed. While this is closely related to MC-237326, it is not identical; it concerns the center of the hearts flashing rather than the outline, and only deals with taking damage rather than both damage and healing.

This is despite the fact that horse/animal hearts do in fact have a dedicated texture in icons.png for flashing in situations like this, rendering it unused.

How to reproduce

1. Cause yourself to take damage, and then heal it
   1. Note that the outlines of the hearts flash white in the process
2. Now cause a horse, pig or other rideable mob with a health bar to take damage, and heal it as well
   1. Note how the background hearts do not, in fact, flash white, even though they did for the player

Other thoughts

icons.png being a predefined texture atlas is problematic for many reasons. I would strongly recommend splitting it into its constituent sprites, as was done with terrain.png, items.png, particles.png and paintings_kristoffer_zetterstrand.png, to alleviate many problems, such as acknowledging the presence of unused icons like these and resolving their unusedness (either through their removal or by giving them use), alongside other problems such as allowing forwards compatibility.