Bats spawn on the surface if the block is covered in a snow layer.

1. start a flat world

2. make an area of stone or other valid bat spawning blocks

3. cover it with snow

4. set time to midnight

Video

Bats are mainly intended to spawn in caves or dark overhangs and shouldn’t spawn on the surface. I believe what’s happening is that the game is checking if a stone block has sky access and it’s seeing the snow layer as blocking the sky so it allows bats to spawn on the stone. I don’t believe this is intended behavior and should not spawn on the surface like this.