Lush caves and dripstone caves will not be generated in the Single Biome world.
Lush cave and dripstone cave biomes can be randomly generated underground in all biomes in the Default world, but do not do so in the Single Biome world.
Probably because the Single Biome world still uses a 2D biome world generation instead of a 3D biome world generation.
Linked issues
relates to 3
MC-236884
In single biome worlds, the terrain doesn't always match the biome (oceans can generate in plains and vice-versa)
Resolved
MC-238805
Single Biome Worlds Broken When set to be a Cave Biome
Resolved
MC-239041
The surface in Single Biome worlds set to use Nether or End biomes consists of grass blocks and dirt
Open
How is this an issue? Dripstone & Lush caves are separate biomes from a plains biome. Having them generate in a single biome world goes against the single world type.