What's the issue you encountered?

Surface layer in Single Biome worlds is almost always the default grass blocks and dirt, no matter what the surface layer for the biome should be.

How can the issue be reproduced?

When creating a world, choose a Single Biome world and select a biome from a TerraBlender-dependent mod. (I have noticed this issue in Oh the Biomes We've Gone, and in Biomes O' Plenty.) If the biome normally generates a surface layer other than the default grass block and dirt, it will generate with grass blocks and dirt in the Single-Biome world instead.

Logs

No response

Mod Version

4.3.0.2

Additional information

Some exceptions to this issue apply, such as Swamp-based biomes sometimes still generating pools of water and mud. This might have something to do with a biome having a surface layer that already exists in vanilla, as I have also seen Lush Caves- and Pale Garden-based surface layers generate properly.
This can cause unintended behavior with certain biomes, such as placed features in the biome being unable to generate on a grassy surface.
If the biome normally has a sandy surface, features may still generate on sand that was generated as part of a shoreline.
There are a lot of other inconsistencies that are probably more related to how individual mods handle the generation of certain features.