I took a pause from my survival world to answer some emails. I did not save and exit, nor did I "pause" the game. I simply opened my inventory and switched tabs. The game sat like this for about 10 minutes. When I switched back to my Minecraft tab, the armor had vanished from the paper-doll in my inventory, and it was also not visible on my character. The armor was all still in my inventory and I still had my armor points above my health. Quitting and reloading the world fixed the problem.

Some additional information:

• The armor was all iron, and all damaged.
• The game "went to sleep" and had to go through the quick-reload screen to resume

Thanks!