The bug

Skeleton horsemen can take damage even if they are immune to damage after spawning from the skeleton trap. For example if you critical hit a skeleton horse or a skeleton horseman once with a sharpness 5 diamond sword while being immune to damage, you will see that crit and enchant hit particles appear meaning that they have taken damage while being immune to damage.

To reproduce

1. Have a diamond sword with sharpness 5
2. Run this command

    /summon minecraft:skeleton_horse ~ ~ ~ {SkeletonTrap:1b} 

3. Quickly critical hit one of the skeleton horses or skeleton horsemen once while they're immune to damage
   → Crit and enchanted hit particles appear after attacking one of them while being immune to damage
4. Critical hit the skeleton horse or skeleton horsemen again
   → The horse or horseman dies