More than 1 biome (rivers don't count) can generate in customized single-biome worlds

Confirmed. Getting Forest / ForestHills when forcing the preset to Plains:

{"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":12.0,"biomeDepthWeight":1.0,"biomeDepthOffset":0.0,"biomeScaleWeight":1.0,"biomeScaleOffset":0.0,"seaLevel":63,"useCaves":true,"useDungeons":true,"dungeonChance":8,"useStrongholds":true,"useVillages":true,"useMineShafts":true,"useTemples":true,"useRavines":true,"useWaterLakes":true,"waterLakeChance":4,"useLavaLakes":true,"lavaLakeChance":80,"useLavaOceans":false,"fixedBiome":1,"biomeSize":4,"riverSize":4,"dirtSize":33,"dirtCount":10,"dirtMinHeight":0,"dirtMaxHeight":256,"gravelSize":33,"gravelCount":8,"gravelMinHeight":0,"gravelMaxHeight":256,"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}

Still not fixed in 14w19a

Bug is also shown in Mesa Plateau-only worlds too.

If set to Ice Plains it still generates Ice Spikes/Mountains.

Confirmed 14w21b

Confirmed in 1.11. Infuriating! I'm trying to create a custom world with only hilly plains and no villages, called Isolation. Please fix this, Mojang! Come on! And maybe make a way to specify multiple types of biomes to generate, instead of just one?

Confirmed in 16w50a.

Confirmed in 1.11.2.

Confirmed in 17w43b.

Confirmed in 17w45b.