As mentioned tangentially in MCPE-54764, right-clicking a stand to place armor on it does not always have the intended effect. For example, in creative mode, if you attempt to place, say, gold armor on an armor stand, then you will also have that armor on your character. When switching armor on a stand to, say, diamond armor, the diamond armor will replace the armor on the stand, but it will also replace the armor currently worn by the player, putting the diamond armor on the player & leaving the gold armor in-hand.

I may have messed up the specifics of this bug, but it is certainly reproducible just by testing different armor types & paying attention to your current armor. Unknown if affects survival in the same way.