Customized world settings Biome sliders change when you re-customize the world

The Biome slider moves unexpectedly when you leave the Customize screen and come back in.

To reproduce:
1. Create New World, world type Customized, hit "Customize".
3. Set the Biome slider to anything after River.
2. Play with some other sliders and buttons, for comparison.
4. Hit "Done".
5. Come back in by hitting "Customize" again.
6. The Biome slider will have advanced in position by a few ticks. Everything else will be where you left it. Also, if it was "Mesa Plateau F" or "Mesa Plateau", it will be in an illegal position, labeled "?".