When moving the biome slider to 'plains' in world customization options, I can exit and re-enter the options multiple times and it DOES stay on plains, however when the world is generated it has rivers, forest, ocean, etc.

Preset used (no unchanged values removed- copied straight from customized preset screen from 'recreate world' menu):

{"coordinateScale":684.412,"heightScale":684.412,"lowerLimitScale":512.0,"upperLimitScale":512.0,"depthNoiseScaleX":200.0,"depthNoiseScaleZ":200.0,"depthNoiseScaleExponent":0.5,"mainNoiseScaleX":80.0,"mainNoiseScaleY":160.0,"mainNoiseScaleZ":80.0,"baseSize":8.5,"stretchY":50.0,"biomeDepthWeight":1.0,"biomeDepthOffset":0.0,"biomeScaleWeight":1.0,"biomeScaleOffset":0.0,"seaLevel":63,"useCaves":false,"useDungeons":false,"dungeonChance":1,"useStrongholds":false,"useVillages":false,"useMineShafts":false,"useTemples":false,"useMonuments":false,"useRavines":false,"useWaterLakes":false,"waterLakeChance":100,"useLavaLakes":false,"lavaLakeChance":100,"useLavaOceans":false,"fixedBiome":1,"biomeSize":8,"riverSize":1,"dirtSize":33,"dirtCount":10,"dirtMinHeight":0,"dirtMaxHeight":255,"gravelSize":33,"gravelCount":8,"gravelMinHeight":0,"gravelMaxHeight":255,"graniteSize":33,"graniteCount":10,"graniteMinHeight":0,"graniteMaxHeight":80,"dioriteSize":33,"dioriteCount":10,"dioriteMinHeight":0,"dioriteMaxHeight":80,"andesiteSize":33,"andesiteCount":10,"andesiteMinHeight":0,"andesiteMaxHeight":80,"coalSize":17,"coalCount":20,"coalMinHeight":0,"coalMaxHeight":128,"ironSize":9,"ironCount":20,"ironMinHeight":0,"ironMaxHeight":64,"goldSize":9,"goldCount":2,"goldMinHeight":0,"goldMaxHeight":32,"redstoneSize":8,"redstoneCount":8,"redstoneMinHeight":0,"redstoneMaxHeight":16,"diamondSize":8,"diamondCount":1,"diamondMinHeight":0,"diamondMaxHeight":16,"lapisSize":7,"lapisCount":1,"lapisCenterHeight":16,"lapisSpread":16}