Gift_of_Names joined the community
This is more of a problem on vanilla's end, and I was only able to find part of the code in Forge pertaining to this (I'm not even entirely sure I was looking in the right place). This is coming from the perspective of a user rather than a modder. Adding/changing dimensions, particularly the Overworld, using data packs in the vanilla style requires non-null seed values for both the dimension generator and the biome provider (which override the actual world seed), which results in worlds with a hard-coded single seed unless the value is changed manually by the user, resulting a non-functional pack otherwise. From what I've examined in OverworldBiomeProvider, it appears if the biome provider seed value is null a default value is returned via an Optional, but doing so results in the aforementioned non-functional pack (which I think probably means the default is also null). I have not been able to track down the handling of the dimensions seeds to know it, so I'm not sure if there is a similar situation there. Therefore, I am suggesting from the best of my knowledge to either have the Optional pass the world seed as its default value or parsing for some sort of key value that sets the seed to the world seed and can be harmlessly passed through the vanilla seed processing.