I started to create a simple mob farm in the overworld. About 70 blocks above sea level. As you can see in the attached screenshots, it's pitch black in there. But no matter what, even at night, hostile Mobs won't spawn.

There is a big platform above this blocking out the light. If I remove lights from that or from the floor below the mob farm, zombies and creepers will spawn there. But never from this self created platforms. Not once.

The height in there is 3 blocks high.