The minecraft:generic.fall_damage_multiplier attribute does not function for several mobs, most of which are affected differently by fall damage.

Affected entities

• minecraft:camel
• minecraft:donkey
• minecraft:goat
• minecraft:horse
• minecraft:llama
• minecraft:mule
• minecraft:skeleton_horse
• minecraft:trader_llama
• minecraft:zombie_horse

It is also worth noting that all entities in the fall_damage_immune entity tag are also technically subject to this issue, as having a value above 0 will not permit them to have fall damage - it may be worth removing this tag from the game and instead giving these entities a minecraft:generic.fall_damage_multiplier of 0 by default instead:

• minecraft:allay
• minecraft:bat
• minecraft:bee
• minecraft:blaze
• minecraft:cat
• minecraft:chicken
• minecraft:ghast
• minecraft:iron_golen
• minecraft:magma_cube
• minecraft:ocelot
• minecraft:parrot
• minecraft:phantom
• minecraft:shulker
• minecraft:snow_golem
• minecraft:wither

How to reproduce

1. Summon a horse

2. /attribute @e[type=minecraft:horse,limit=1,sort=nearest] minecraft:generic.fall_damage_multiplier base set 0

3. /tp @e[type=minecraft:horse,limit=1,sort=nearest] ~ ~10 ~

Observed: The horse incurred fall damage.
Expected: The horse should not suffer fall damage.