I found this when trying to create a void world and flat world using a custom datapack on the pre-releases for 1.16.

When creating a custom dimension using a datapack, the "biome_source": "type": can only be "minecraft:vanilla_layered" or "minecraft:the_end", if you have multiple custom dimensions in the same folder, say under "custom", they both will not load if one is not using the right settings.

Also when changing the "generator" type, "noise" and "debug" works, however "flat" does not.

Furthermore, copying data from a worldgen_settings.json that is pulled straight from a superflat
"The Void" preset will not work, even though it is from the "generator" tag down.

The only information I have to go off of is RainWarrior's github examples: _https://github.com/RainWarrior/_ and the official minecraft wiki page: _https://minecraft.gamepedia.com/Custom_

I have attached some examples, feedback is appreciated.