So, despite already looking around and trying to spread the word, getting comments deleted even, the terrain generation for single biome customization is broken despite the "Working as intended" from any "developers" or "moderators." As this is a current and quite devastating issue, this is why competent modders are quite needed for games. The single biome doesn't actually generate the single biome as wanted nor intended, but rather generates a "single biome."
On a technicality it does generate only the one biome, but in all actuality it generates a normal plains world, just with the biome specific structures only, which actually breaks how the world generates in many ways; one of which being the realism/immersion, and intended usage of the "Single Biome" world type. If you won't fix it, remove it. As it isn't working properly, or rather as we want, then it is simply a broken feature that should no longer be a feature if it's a so called "working as intended feature."
Oh, but feel free to delete this too, as I feel you probably will, or NPC/Bot reply to this as "Feature is working as intended" which is actually utter nonsense to just silence your paying and active community.