Armor on the armor bar disappears when walking through nether portal even though you are wearing armor

When you walk through a nether portal with armor, your armor on the armor bar disappears even though you are still wearing armor. Evidence: http://www.youtube.com/watch?v=mGSaHxaDwb8&t=5m33s . You can see that they had their armor on before walking through the portal and after it disappears and after a few mins/secs it reappears.