In 1.17.1, horses summoned with the command start out with 53 health.  They are immune to the first hit and instead reset to the correct health when hit for any amount of damage.

Steps to repeat and detect/prove.

Make a small pit.

Summon a horse variant (Llama, Mule, etc.) in it.

Execute the command:

"/data get entity @e[type=llama,limit=1,distance=..8] Health"

Observe the health is 53.0.

Hit the horse with a torch.. or a fully enchanted netherite sword...

Execute the /data command again.  Observe the hit points are set to maximum normal.

Subsequent hits damage the horse variant.  This can be verified by executing the /data command again after each hit.

Note; It looks like Horses were broken in 1.16 but they are fixed now.   We suspect the problem is with the AbstractHorseClass instead of the Horse Class (which is fixed).