If you right-click to fast-swap your armor (for example, from chestplate to elytra), the game will delete piece of the armor you were wearing.

Steps to reproduce:

1. Put on the first piece of armor.
2. Then take the second piece of armor to your hotbar (this armor must be the same type as the first one).
3. Right click with the slot of the hotbar, you have placed the second piece of armor to swap with the first piece of armor.

Expected result:
Armor swaps and you have the first piece of armor in your hotbar slot.

Observed result:
Armor piece you were wearing disappears.