When you access an inventory (not your own, but f.e. chest, enderchest) and you use the /replaceitem command with a armorslot, it will be replaced, but as a ghost item. This item doesn't appear and in case of an armor piece it doesn't give you the protection. In gamemode 2/0 you can click in the armor slot and then the item will appear. In gamemode 1 you can't get rid of it.

To reproduce:
1. Put this command in a commandblock and create a delay with repeaters: "replaceitem entity @p armor.chest minecraft:diamond_chestplate"
2. Place a chest
3. Press a button to activate the delay, rightclick the chest before the commandblock gets powered
4. Check if the slot has been replaced (It hasn't)
5. (If you're in gamemode 0/2 you can make it appear by clicking on the slot, if you're in gamemode 1 you have to either switch to gamemode 0/2 or reload the world/relog)

A picture for the setup is attached