Inside a region file, each chunk has an "InhabitedTime" value which is supposed to increment each tick a player stands in a chunk.

I've created a 21w40a world and explored it a bunch in creative and spectator mode. Upon examinating the region files using NBTExplorer, I've been able to constate that none of all the stored chunks Level.InhabitedTime values have incremented, they are all still at 0, breaking Local Difficulty.