Hello, so how exactly do I fix this issue in 1.17, because I still have it. A small walkthrough would be very helpful! Thank you.

Hey, I'm the author of that plugin. I made the plugin because Spigot didn't support custom dimensions in 1.16 at all, now it seems to support them, though I haven't done a lot of testing.

The dimension folder name doesn't break the namespaces as they are treated separately. In my plugin I also added an option to rename dimensions in Bukkit. The name Bukkit uses can be anything, the plugins will see that name and the world folder is named after that, while the key of the dimension is used by the vanilla server and depends on the datapack.

If you're still having this issue, add "type":"minecraft:overworld" before the "generator" key. I haven't tried the datapack but I noticed this. The flat type is kind of misleading, it's different than the dimension type. The dimension type has a lot of settings that you had initially but if you want to alter those then you will need to create a separate dimension type according to the Wiki, I'm not sure why does it work in vanilla that way.

It seems other people have had this problem with Spigot in the past as well.

Someone made a plugin to fix it in 1.16.5.

https://www.spigotmc.org/resources/1-16-custom-dimensions.83731/

{    "generator": {      "seed": 0,      "type": "minecraft:flat",      "settings": {        "biome": "minecraft:mushroom_fields",        "lakes": false,        "features": false,        "layers": [          {            "block": "minecraft:grass_block",            "height": 1          }        ],        "structures": {          "structures": {}        }      }    }  }

I changed the .json and got rid of the "type" segments. Still does not work in Spigot.

No the dimension type is used for "type", the first dozen lines of your screenshot

The "dimension_type" folder is only for custom dimension types. I am using "minecraft:flat" as my dimension type shown in the .json file in the image above that generates my custom Dimension.

The "name" key that you say doesn't seem to be used is what is used to create the namespace. When loading this datapack, Vanilla will create a "dimensions/kingdoms/spawn" folder. Datapacks rely on the namespace "kingdoms:spawn" (in my case) to access the custom Dimension.

Spigot does not create a "dimensions/kingdoms/spawn" folder. It instead tries to create a folder called "world_kingdoms_spawn" in the root Minecraft directory. This breaks the namespace system, making it invalid when trying to access in game.

The solution is to make a dimension_type folder like the wiki says and vanilla does

I'll dig into it more when I wake up. It is 4am. If I can get it running in Spigot, I will let you know my solution.

Yeah sorry I didn't catch the caves the first time round, but I still think the pack is technically invalid even though it does work on vanilla

Oh! Good to know!

I will read up on that! Thanks!

Although, I got this to load into a Vanilla 1.17 server just fine using the same methods as before. Here is a screenshot of that:

(Same Datapack, be sure you are using CavesAndCliffsPreview.zip datapack with it too, like I mentioned above.)