When i go in the nether or exit the nether my armor bar dissapears and the only way to fix it is to take off one piece of armor and put it back on.

Put the summary of the bug you're having here

Here is how to reproduce the glitch
1. Be in survival mode and have armor on

2. Go to the Nether and stay there for maybe 5 min

3. Exit the nether
That should do it and if not maybe make 2 portals (overworld) that connect to the same gateway portal in the nether.