Traveling to Nether via portal "unlocks" world difficulty

On a world that is locked to a certain difficulty (for instance, "Normal"), if the player is too enter the Nether via a portal, the difficulty will "unlock" and the player can change it again. This can be fixed by exiting the current world and re-opening it. The bug may be caused by an error with keeping the locked difficulty setting through dimensions, so it's possible it may be the same case for the End as well.