So when I go to the Nether the Armour meter just disappears. It also happens in the Overworld when I leave the Nether. It can be fixed by pressing Esc and pressing Back to Game.
Environment
I don't get what the Environment means but I got the Overworld and Nether.
Confirmed. The same thing is happening to me.
Pressing ESC and then Back to the Game does not work on a Multiplayer Server. Reconnecting is required to get it to reappear.