Mobs don't spawn above ground.

So in a survival world that I had I created an above ground mob spawned out of netherack. Went afk for a wile then when i came back after a couple hours I realized i had no items in my chests and no mobs on the ground or in the spawned itself. So I ran an experiment to make sure i changed the block on the ground where the mobs would spawn ad when it didn't work I was frustrated. Because it took me 30 minutes or so to make the farm. So I did some reasarch to make sure. Mobs can spawn on any block and below the sky which I had both. So I figured it was a bug.