The armor reduces the damage from the wither effect even without the protection enchantment.
Steps to reproduce:
- Put some unenchanted armor on
- Give yourself the wither effect or get attacked by a wither/wither skeleton.
- 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.