Flat World's use Old Height Levels

I am not sure if this is Truly a Bug or a Feature, but please fix the Height Level of the Flat World's. It uses the old 0 to 256 Y Levels and not the -64 to 320 Y Level. Please fix this, it just disappointed me a little bit. But as the Younger 9th Sceptre of Minecraftia, I should say Good Luck on everyone!

 

Please Fix this. If it's a Feature, then please make it possible.

Thank you