When you press the "Defaults" button in the customized world generation settings, a message appears asking you if you are sure about this. At the same time all GUI elements in the background are being disabled (probably to prevent
MC-24390 from happening). If you however resize the game all buttons will be enabled.
- Open the customized world generation settings
- Change a value
- Click on the "Defaults" button
- Resize the game
→ All buttons are enabled again