The bug
When creating a superflat world with only one block, villages will spawn as if the world was its default of 4 blocks. This creates a strange floating village.
How to reproduce
Create a new world, superflat, with the preset code:
minecraft:grass_block;1;village
Find a village in that world
Observe
Can no longer reproduce in 1.13.1. Someone else please confirm that this is fixed.