The surface in single biome worlds set to use nether or end biomes consists of grass blocks and dirt

The bug:

Using a biome from the wrong dimension in the wrong type of buffet world causes the wrong surface to generate.

• Using a nether biome, end biome or The Void in a single biome world causes a surface made of grass blocks and dirt to generate.

• [Before 1.18-pre1] Using an overworld biome, end biome or The Void in a caves world causes no surface to generate on terrain surfaces (basically MC-237181, but it no longer happens in nether biomes), but instead, the surface generates under the bedrock ceiling, like in MC-238930. It is only made of netherrack, except when using specific nether biomes, which aren't affected by this bug in cave worlds.

• [Before 1.18-pre1] Using any biome in a floating islands world would technically cause a surface of end stone to generate, but the (very few) islands in floating islands worlds don't have surfaces at all in 1.18 snapshots (MC-236723) so it cannot be reproduced.

Affected biomes:

• Nether Wastes

• Soul Sand Valley

• Crimson Forest

• Warped Forest

• Basalt Deltas

• The End

• Small End Islands

• End Midlands

• End Highlands

• End Barrens

• The Void

How to reproduce:

1. In the world creation screen, go to the "World" tab.

2. Change the world type to "Single Biome."

3. Click on "Customize."

4. Select one of the above biomes.

5. Click on "Done" and create the world.

Expected result:

Depending on the selected biome, the surface of the terrain in a single biome world would consist of blocks that can normally be found in that biome (i.e. netherrack/soul sand/soul soil/nylium/basalt in nether biomes, end stone in end biomes, stone/air in The Void).

Observed result:

If the selected biome does not normally generate in the Overworld, the surface of the terrain in a single biome world still only consists of grass blocks and dirt.

Cause:

The new surface building system is based on a huge condition tree in the noise settings of a dimension. The default noise settings minecraft:overworld (which is only used for the overworld) is used for single biome worlds, minecraft:caves (with its condition tree copied over from minecraft:nether) is used for caves worlds, and minecraft:floating_islands (with its condition tree copied over from minecraft:end) is used for floating islands worlds. Each set of noise settings has its condition tree tailored just for its specific dimension, so using a biome that the tree has no check for causes the wrong surface to generate.