meyfa

I don't think that is correct; "minecraft:worldgen/biome" isn't present in my registries.json file, either (even when using the "--all" flag with the command).

I'm sorry, I should have been more specific. I would expect there to be a top-level key named "minecraft:biome" that looks as follows:

{
  "minecraft:biome": {
    "entries": {
      "minecraft:badlands": {
        "protocol_id": <id>
      },
      ...
      "minecraft:plains": {
        "protocol_id": <id>
      },
      ...
    },
    "protocol_id": <id>
  }
}

In essence, it should list the contents of the biome registry - meaning, all biomes registered within the game and their respective protocol IDs - the same way it is done for items ("minecraft:item"), potions ("minecraft:potion"), and all other other registries that are present within the registries.json file.

The presence of "minecraft:biome" as an entry of other registries is not helpful, as the list of biome protocol IDs is nowhere to be found.