/difficulty resets after reloading the map!

In singleplayer, after using the /difficulty command and reloading the world, the map difficulty resets to the difficulty selected in your menu.

I'm not sure if this is a bug or just a (missing) feature, however, it should definitely be fixed as it makes the /difficulty command completely useless. I think it would be nice if the game saved it as a map specific difficulty which would always be used for that map.