1.16 Pre-release 2
Java Freshly updated (8.0.051.16)
Custom Worlds, World generation
Created a new world with just 1 layer of Bedrock instead of default 2x dirt AND 1 layer of grass_block to make a flat world for testing redstone contraptions in 1.16 pre-release 2
create a new world with the following settings:
Go to customize and pick the default preset: bedrock (1 layer); 2 layers of Dirt and 1 layer of Grass;
Delete the dirt and grass layers, only Bedrock left!
Create and open the world.. you'll see that the grass AND dirt layers DID spawn in.