Overworld superflat preset no longer generates with terrain decorations

The Overworld superflat preset no longer generates with terrain decorations such as grass, trees
and water lakes. These settings are missing from the Overworld preset.

If these are added manually by importing them from a 1.15.2 preset, they do not generate.

1.15.2: minecraft:bedrock,59*minecraft:stone,3*minecraft:dirt,minecraft:grass_block;minecraft:plains;pillager_outpost,village,biome_1,decoration,stronghold,mineshaft,lake,lava_lake,dungeon

1.16 pre-2: minecraft:bedrock,59*minecraft:stone,3*minecraft:dirt,minecraft:grass_block;minecraft:plains

Attached screenshot shows the missing decorations. It also has inappropriate structures as shown in MC-180603.