The Overworld preset for flat worlds generates the wrong structures for the world's biome (plains for example).
Some structures (which would not normally generate in a plains biome) do generate on a flat with this preset like Jungle Temples or Ocean Ruins.
Heres more examples with the screenshots below:
Environment
Recreate the bug?
1st: Start up a new world with the flat world type with the overworld preset.
2nd: Create the new world.
3rd: Fly around in creative and see the chaos that is this bug.
Thank you for your report!
However, this issue is a Duplicate of MC-180603.
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