When creating a Superflat world with parameter "biome_1" structures from any biome spawn in this world.

What I expected to happen was...:
Only structures from plains biome, such as: villages, mineshafts, trees etc.

What actually happened was...:
All of the above structures was spawning, but also desert and jungle temples, igloos, witch huts, shipwrecks, ocean ruins

Steps to Reproduce:
1. Create new Superflat world on 20w18a
2. Add parameter "biome_1" or use "Overworld" preset
3. Enjoy