When loading a worldgen datapack that affects the overworld dimension file at the first launch of the server, the changes to the dimension file will not be reflected in generation until the server is restarted. I've attached a datapack which can be used to test this.

Steps to reproduce:

1. Start a blank server with the only file being Terralith v2.0.3.zip in /world/datapacks/
2. Notice that biomes are affected, but NOT the dimension (custom biomes won't show up)
3. Shut down the server.
4. Start the server again.
5. Travel some distance and observe the harsh chunk borders against modified terrain.

Expected behavior with this datapack:

You should spawn in the `terralith:yellowstone` biome immediately upon starting the server for the first time.