Luke Murray

—  In the attachments you've provided, I can see that you're using modifications
No, I wasn't (unless you count the vanilla shader). I had just created a separate instance with 21w13a to test shaders, no mods.

It appears the BlusteryDay shader checks the block atlas to check if the block texture uv is correct, to ensure it only affets leaves, etc and not also other rendertype_cutout blocks. This means that if you have a non 16x16 texture, it will offset the uv from the expected loc for 16x16, and skip the if statements. However, as of now, there is no better way to check the block than just using the texture id, which (as seen here) is rather fragile

If we take a look at the world data, although there is the folder data for the nether, there is no link to it in the level.dat -> Data -> WorldGenSettings -> dimensions, only to the overworld. This occurs (I have seen) when upgrading a world with custom dimensions (modded or data pack) to a newer game version, in which the game not only removes the custom dimension links, but also removes the nether (and end if present). Once the link is gone, the game will not make you back for you. A work around is to manually add back the link in the level.dat by copying from an older backup (though an entirely different world will work, you will just have to manually copy back the seed). To do this, you just need NBTExplorer or similar nbt editing software. This will at least fix the nether/end. As for the datapack upload for custom dims, I don't know;