The bug:
Armor pieces do not have default attribute modifiers component, although they have default attribute modifiers. Weapons and tools have default attribute modifiers as well, but they also have a component with said attributes. Due to this you can't remove attributes with an empty component, and checking for lack of it passes.

Steps to reproduce:
1. Get armor piece and a weapon items with component minecraft:attribute_modifiers=[]
2. (x)See that in description of given items, weapon doesn't display any attribute modifiers, while armor piece does;

Another way:
1. Get armor piece and a weapon
2. Run /execute if items entity @s weapon.mainhand *[minecraft:attribute_modifiers=[]] on both of them
3. (x)See that armor piece passes the test, while weapon doesn't