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
(see screenshots)
Reproduce:
create a new world with the following settings:
Flatworld
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.