The damage of the wither boss skull shell is unstable (no projectile damage on direct hits)

Looking at the wither_skull.json entity file, the skulls do not do any projectile damage at all (no impact_damage). It looks like all of the damage is from the explosion, so the inconsistency you’re seeing might be explained by general randomness in explosion damage, or by how close you are to the center of the explosion.

Another factor could be difficulty level. Were both videos made with the same difficulty level?

Yes, both videos have the same difficulty level, the difficulty level of both videos is Normal.

After thoroughly testing and looking at the vanilla behavior files, I can say that the randomness in wither skull damage is indeed due to randomness in how close the center of their explosion is to the player. If you remove all randomness from their trajectory with a behavior pack then they always do the same amount of damage. Also, if you set them to be summonable and summon them on yourself, the damage amount is always the same.

However, wither skulls should always do 8 projectile damage (4 hearts) to mobs they hit directly in vanilla, and then only do explosion damage to mobs nearby the point of impact, as in Java Edition (MC-178576). The reason they do not deal the stable projectile damage is that the projectile component in wither_skull.json and wither_skull_dangerous.json has no impact_damage section within the on_hit parameter. Compare this to the small_fireball.json, which correctly gives blaze fireballs 5 projectile damage.

Ghast fireballs have the same bug, which I have reported at MCPE-153740.

affects 1.20.12

This issue affect 1.20.32

This issue affects version 1.20.40

This issue affects version 1.21.21