Creating a world with defined generatorSettings in the WorldCreator object does not affect the outcome of the world. This did not occur in 1.15.2.

It could be a problem with the properties settings when creating a world, or it could have to do with the JSON itself has changed and the docs have not updated. That's just my speculation though.

    WorldCreator creator = new WorldCreator("test");
    creator.type(Objects.requireNonNull(WorldType.valueOf("FLAT")));
    creator.generatorSettings("{\"layers\": [{\"block\": \"air\",\"height\": 1}],\"biome\": \"plains\"}");
    Bukkit.createWorld(creator);