The armor reduces the damage from the wither effect even without the protection enchantment.

Steps to reproduce:

1. Put some unenchanted armor on
2. Give yourself the wither effect or get attacked by a wither/wither skeleton.
3. You will see that your health is barely decreasing with the armor on

In pre-1.19, the armor before didn't reduce damage against the wither effect without the protection enchantment. It would've needed the enchantment in order to reduce damage from the effect.

I think this issue is related to the armor durability damage issue since it takes durability damage so it would protect against the effect without the proper enchantment.

MCPE-157700

Video - Armor on

Video - Armor taken off and later put on