When using the "type":"minecraft:multi_noise" for generating a custom world, it also takes a "biomes" array in the json file.

However, only the first one biome in that array will actually be used to generate the world. All other biomes never get generated.

I have uploaded an example datapack to use, which was what I am using for testing. You can get to the dimension using:

/execute in custom:test run tp @s ~ 90 ~

EDIT:

I was mistaken when I stated "first biome in array". But after more testing, I have discovered it will generate 1 where the parameters are above 0, and 1 biome where the parameters are below zero. It seems to be the biomes "closest to zero" that are chosen.

I have uploaded two new datapacks to show this off. One will only spawn plains and deserts, and one will spawn jungles and frozen oceans. But for both of them, all four of those biomes are in the "biomes" array, and no parameters are the same.