Mobs don't spawn in the Overworld, and the Wither won't spawn.

In the Overworld, mobs won't spawn. Play several hours, built a platform 30 blocks out for mobs to spawn. Neither hostile nor passive mobs spawn naturally. (Btw it's a void world - friends and I wanted to play a world called Vanilla SkyBlock on CurseForge.) Played for hours now. For some reason, mobs in the Nether will spawn. Went to the Nether roof, tried to spawn the Wither in, didn't spawn. Brought it to an open area, still didn't spawn. ("And I said: WHY-e-i-ei-ooo")

EDIT: After more testing, I think I know why. Even during the night, both the Client Light and Server Light, for the sky, is 15 during both the day and night. Since mobs in the Overworld spawn in low light levels, they won't spawn. However, since mobs in the Nether don't take light levels into account, they do spawn. The sky light level can be changed if you block the sky with a series of blocks.