When the player creates a single-biome world and uses a Nether biome for it, grass blocks will generate on the surface despite not being normally present in Nether biomes.

To reproduce:

1. Create a single-biome world using a Nether biome (Nether wastes, soul sand valley, warped forest, crimson forest, or basalt deltas).
2. Notice that the surface is covered in grass blocks.

Past bug reports involving stone and water being generated in single-biome worlds using Nether biomes have been marked as WAI (works as intended). While I suspect this is a related issue, I do not think this issue WAI. This is because other biomes, like desert and badlands, can generate grassless single-biome worlds, and because single-biome worlds using Nether biomes did not generate grass pre-1.18.

Below I have attached pictures of a basalt delta world and a warped forest world. In the basalt delta picture, the ground is grass, but the unique terrain of the basalt delta is able to generate. In the warped forest, the ground is grass and no giant warped fungi or warped vines are able to generate, most likely because they can only generate on netherrack.