It's unclear whether a remix album should use the same release group as the original album or get its own release group. I think that the closest the guidelines get is the "Other situations" section of https://musicbrainz.org/doc/Style/Release_Group (emphasis mine):

There are a number of situations where there is currently no consensus about whether releases should be grouped together or not. This includes the same album sung in different languages, re-recorded albums and remixed albums. Be aware that attempting to group these releases can be quite controversial. If in doubt, these releases should probably be in separate release groups.

As seen on https://community.metabrainz.org/t/should-different-remix-singles-be-merged-into-one-release-group/174541 (and on many edits), experienced editors still interpret these guidelines in different ways. It feels like a lot of time is wasted debating this in edit comments and that remix releases sometimes get moved back and forth between the album release groups and their own release groups.

I think that much of the confusion stems from the "would you say that you have the album if you possess this release?" test for release groups. (I'm not sure if this appears in the official documentation or style guidelines, but it seems to be widely accepted.) I've seen different remix albums for which the answer might be different:

• Sometimes the remix album contains no overlap with the original album, e.g. every track is a remix of one of the original tracks, and none of the original tracks are included.
• Sometimes the remix album contains an original single and one or more remixes of that single.
• Sometimes the remix album is just one track that's a remix of one of the original tracks.