The minecraft:fall_damage_multiplier attribute does not work when riding donkeys, zombie horses, mules, llamas, camels, trader llamas, horses, or skeleton horses. However, it works when riding any other mob.

How to reproduce

1. /attribute @s minecraft:fall_damage_multiplier base set 0

2. Spawn a camel
3. Ride the camel

4. /execute on vehicle run tp ~ ~10 ~

Expected result

You would not take any fall damage, as your minecraft:fall_damage_multiplier attribute is set to 0.

Observed result

You took fall damage despite your minecraft:fall_damage_multiplier attribute being set to 0.