When you create an enchantment that is intended to work on multiple pieces of armor (and stack, similar to how enchantments like Protection work), which uses attributes, the effects of the attributes are unable to stack because they have the same UUID. I believe this is a separate issue from other posts revolving around attributes on armor not stacking because this issue cannot be fixed without creating a separate enchantment for each armor piece, which doesn't seem to be "working as intended".