Mob spawn in full daylight at y=255

I was experimenting with the custom presets and generated a world where the terrain generates right up to the build limit. Most of y=254 is completely filled with scattered pockets of terrain at y=255.

On these scattered pieces of terrain at y=255 mobs can spawn in full daylight and don't start burning until they drop to y=254.

I didn't expect mobs to spawn in full daylight at this y level.