The bug

When mobs or the player have jump boost and they fall as high as they jumped they won't take fall damage. This is not the case for horses which take fall damage.

How to reproduce

1. Summon a horse with a generic.jump_strength attribute of 1.0 (the maximum obtainable in survival). As with all horses, it will have a generic.safe_fall_distance attribute of 3.0 and a minecraft:generic.fall_damage_multiplier attribute of 1.0.

   /summon horse ~ ~ ~ {Tame:1b,SaddleItem:{id:"saddle",Count:1b},Attributes:[{Name:"minecraft:generic.jump_strength",Base:1.0d}]}

2. Give it Jump Boost 4 (the maximum supported under MC-10755)

   /effect give @e[type=horse,limit=1,sort=nearest] jump_boost infinite 3

3. Mount the horse and perform a fully-charged jump (on a level surface)
   The horse takes damage

In other words, as of 24w06a, the core issue here is that the new generic.safe_fall_distance attribute is always 3.0 for horses instead of scaling with their base jump height (preliminary testing using /summon horse and /attribute @e[type=horse,limit=1,sort=nearest] minecraft:generic.safe_fall_distance get confirms that it does not scale). It would be handy if the safe fall distance would "intelligently default" when a horse is summoned with only the jump strength specified (in line with my suggestion on MC-268106) but this is not strictly required to mitigate the survival impact of this bug.