Difficulty unlocks after going through nether portal

[tbfox]

whenever I go through a nether portal my difficulty unlocks. Once unlocked I can change the difficulty but the game will not actually change behavior until I go through the portal again. If i want to lock the difficulty I need to exit to the title screen and enter the game again.

On the one hand this is super useful for when you are trying to catch a ghast and die in the lava too many times to count but is probably not the intended behavior.