Some Zombiefied Piglin spawn with HP 20 / 90 for example. See picture bellow.

When aggro, those mobs don't jump down from height they would take damage from. If you force them to jump down with trap doors, they are spinning in circles for around 2 seconds after landing on the ground before they continue pathfinding again.

Those Zombiefied Piglins, spawning with regular HP 20 / 20, jump from any height when aggro.

I suspect the problem to be that they are already spawning "injured", which may prevent them from voluntarily taking additional fall damage.