I chose the default preset and added stronghold;mineshaft;dungeon (to get floating structures) after the pre-set settings, clicked use preset and it erased my changes. I tried with other combinations and other presets and it happened again.

What I expected to happen was...:
I expected it to save the changes and generate the world I wanted.

What actually happened was...:
It reverted to defaults and gave me a standard super-flat world.

Steps to Reproduce:
1. Make a new world
2. Set to super-flat and go to customize
3. Change preset settings
4. Add stronghold;mineshaft;dungeon after the settings in any order on any preset
5. Generate world
6. Try using Eye of Ender to find Stronghold, it's disabled. No other structures generate except villages or other structures if they're pre-programmed.