Biomes use the minecraft:replace_biomes component targeting the Nether generate incorrectly. Instead of smooth, noise-based transitions, other shapes such as diagonals and lateral bands appear.

Demonstration

The attached screenshot illustrates the problem. The custom biome definition is nothing special:

{
	"format_version": "1.26.10",
	"minecraft:biome": {
		"description": {"identifier": "bug_nether:nether_custom_biome"},
		
		"components": {
			"minecraft:surface_builder": {
				"builder": {
					"type": "minecraft:overworld",
					
					"foundation_material": "minecraft:grass_block",
					"mid_material": "minecraft:grass_block",
					"top_material": "minecraft:grass_block",
					"sea_material": "minecraft:water",
					"sea_floor_material": "minecraft:grass_block",
					"sea_floor_depth": 4
				}
			},
			
			"minecraft:replace_biomes": {
				"replacements": [
					{
						"dimension": "minecraft:nether",
						"targets": [
							"minecraft:soulsand_valley",
							"minecraft:basalt_deltas",
							"minecraft:hell",
							"minecraft:crimson_forest",
							"minecraft:warped_forest"
						],
						"amount": 0.6, "noise_frequency_scale": 4
					}
				]
			}
		}
	}
}

Reproduction

The demo screenshot and code are taken from the behavior pack in the attached world for convenience. This world can be opened (in Minecraft Preview) for inspection or unzipped to access the code associated with such.