o/

I've done quite a bit of testing, and it looks like biomes and dimensions don't actually work. I looked through the code, and discovered that the biomes and dimensions lists are private, where the other entries are protected, could that be the cause?

https://github.com/alcatrazEscapee/ore-veins/blob/1.12/src/main/java/com/alcatrazescapee/oreveins/api/AbstractVeinType.java

Script used:
https://pastebin.com/iKqytVM0

That script generates oreGold in the Overworld, dimension 0, but not in dimension 2, suggesting the dimensions_is_whitelist property is actually false by default?

Or it could be user error ofcourse :P

You have an extra space in the "dimensions " key.
Correct vein:

{
  "gold_ore": {
    "type": "sphere",
    "ore": "ore:oreGold",
    "stone": "minecraft:stone",
    "rarity": 20,
    "min_y": 40,
    "max_y": 70,
    "density": 800,
    "vertical_size": 4,
    "horizontal_size": 6,
    "dimensions": [
        2
    ]
  }
}

Woops, thank you. And thanks for putting up with my many many issues :)