If you are pick up an item in your inventory, then click the chestplate button to open the UI, it then deletes the item held. I found this when trying to throw an item out of my inventory while trying to open the cosmetic armour ui.
Happens in version 1.16.5 v5a

Are you in creative mode?
I just tried it, with a clean install of CosmeticArmorReworked-1.16.5-v5a, only when you are in creative mode, items are deleted, and when you are in survival mode, items are thrown properly.