When loading datapacks that modify the world generation of existing dimensions (Overworld, The Nether and The End) and custom dimensions, in version 1.17.1, an expection of the type "Unregistered dimension" rises up:

This doesn't happend in vanilla 1.17.1, it generates the proper dimension directory for custom dimensions and correctly overwrites the "vanilla dimensions" generation if specified in the .json files of the datapacks:

I uploaded an example of a datapack that modifies the existing dimensions, with custom features, terrain generation and structures: NormalDimensions-Example.zip and a datapack that generates a new dimension (named skylands): CustomDimension-Example.zip as examples for verifing the Exception.