Only NoAI-horse in combination with NoAI-passenger does not float and is knocked back in case of damage

You can spawn a horse with out AI with

/summon minecraft:horse ~ ~ ~ {NoAI:true}

and the horse has no gravity and if you hit it, it will not move.

But if you spawn a passenger on it with this command:

/summon minecraft:horse ~ ~ ~ {NoAI:true,Passengers:[ {id:"minecraft:husk",NoAI:true} ]}

The horse will be falling down, if it spawns in the air and if you hit it, it will move in a direction.

This does not happen to all other rideable (with commands) creatures:

/summon minecraft:sheep ~ ~ ~ {NoAI:true,Passengers:[ {id:"minecraft:husk",NoAI:true} ]}
/summon minecraft:cow ~ ~ ~ {NoAI:true,Passengers:[ {id:"minecraft:husk",NoAI:true} ]}
/summon minecraft:llama ~ ~ ~ {NoAI:true,Passengers:[ {id:"minecraft:husk",NoAI:true} ]}
...