The bug

When using a block that has states in default_block or default_fluid, you are required to give it Properties, otherwise an error will show and the dimension will not load. It is expected to treat missing Properties as the default, empty properties.

Update: You now also need to specify all the properties for that block or fluid. For example

{
  "Name": "minecraft:water",
  "Properties": {
    "level": "0"
  }
}

 Original Description

I accessed a base worldgen_settings file via Github (https://gist.github.com/RainWarrior/2c32848aad78910a3ca148b82c91f7c2) and importing it into my world worked great, as I was able to use the teleport command to access the dimension.

However, when accessing said file and making a minor change to the following setting:

"default_fluid":

from "minecraft:air" to "minecraft:water" I found it prevents the dimension from existing at all, not appearing in the dimensions folder, no teleport options, and when settings are exported, the dimension is not in the exported settings file. This also applies to changing the "default_fluid" to "minecraft:lava"

other variables I have changed the "default_fluid" to include; "minecraft:stone" "minecraft:sea_lanterns" and "minecraft:crimson_stairs", as well as changing the sea level. I was able to access the new dimension and see the generation in the first two, although "crimson_stairs" also resulted in the custom dimension not generating.

I have not tried a full list of blocks, but still have found that some blocks can result in the prevention of dimensions, although the primary bug is that the games 2 fluids both prevent generation when specified as the "default_fluid"