"Superflat Customization" screen resets every time you open it and does not show previously saved settings

The bug

When creating a new world, if something like the superflat preset is changed and saved, going back to the same page does not show the previous configuration, but resets to default - however, the settings are still saved and if you click "Cancel", then generate the world, the original settings are used.

How to reproduce

1. Start creating a world
2. Set the world type to superflat and change the preset (I for example used minecraft:bedrock,99*minecraft:white_concrete;minecraft:forest)
3. Click Done
4. Open up the same screen again
5. The settings have been reset