Mobs not spawning in world spawn area

Hostile mobs do not spawn in the world spawn area, but spawn fine in other areas.

Tested by creating a flat-world with only bedrock, and making two spawnable areas 10x10 blocks, one located at world-spawn (marked in the picture by diamond block, identified using compass), and the other a few blocks away.

Standing an equal distance of around 30 blocks from both, it is quickly apparent that no mobs spawn near the world spawn.