In 1.13.2 (and 1.14 snapshots), im unable to create a custom superflat world because it wont save the changes. 

What I expected to happen was...:
I expected to get custom sized villages, mineshafts and dungeons/strongholds.

What actually happened was...:
The game doesn't save my preset when i modify it manually.

Steps to Reproduce:
1. Go to new world and select Superflat world.
2. Go to "Customize", "Presets" and manually type "village(size=1 distance=9),mineshaft(chance=0.1)
3. click "Use preset", "Done" then go back to customize preset and you'll see that the changes are no longer there.

4. You can create the World after manually editing the preset but is not going to work as intended.