Some Zombified Piglin are spawning with more than 20 maxHP and are avoiding fall damage.

Some Zombiefied Piglin spawn with more than 20 max HP. They spawn for exmple with HP 20 / 70 or HP 20 / 90.

When aggro, those mobs don't jump down from height they would take fall damage from. If you force them to jump down with trap doors, they are spinning in circles for around 2-3 seconds after hitting the ground until they continue pathfinding again.

I suspect the problem to be that they are already spawning injured, which may prevent them from voluntarily taking additional fall damage. When they are healed up with Splash Potions of Harming those mobs jump of the edge again.

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

I attached a video showing that the ones avoiding fall damage can be healed above HP 20 with some Splash Potions of Harming (Taking up to 10 hits). 

I also attached a screenshot showing around 60 HP with data get entity after using Potions of Harming.