When creating a new superflat world with the overworld preset selected, jungle temples, igloos, desert temples, shipwrecks, underwater ruins and witch huts (probably also mansions, monuments...) appear all over the place although the biome is plains, these structures can not be located with the /locate command.

To replicate: open Minecraft 20w19a, create a new superflat world with "Overworld" preset, then fly around to see the structures.