The bug
Grass / flowers / snow / trees are placed incorrectly in superflat world.
Reproduce
Flower Forest
minecraft:bedrock,63*minecraft:dirt,minecraft:grass_block;minecraft:flower_forest;biome_1,decoration,lake,lava_lake
Mushroom Fields
minecraft:bedrock,63*minecraft:dirt,minecraft:mycelium;minecraft:mushroom_fields;biome_1,decoration,lake,lava_lake
Bamboo Jungle
minecraft:bedrock,63*minecraft:dirt,minecraft:grass_block;minecraft:bamboo_jungle;biome_1,decoration,lake,lava_lake
Related issues
is duplicated by
Attachments
Comments

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This issue relates to MC-610.
@unknown say:
Super flat behaves different so make another ticket for that.

In 1.16.1 and 20w29a.

How is this not just a duplicate of MC-610?

MOJANG decided to split the issue.