I’ll start right with an example:
Blue Sky Black Death has an album called “Noire”. In the Noire release group you can find Noire and Noire DLXE which is a delux version containing bonus remixes.
Then there is also an additional release group called Noire + Violet which only contains some of the remixes from Noire DLXE, but no original tracks from Noire.
I’d still argue it should be in the same release group.
Instrumental versions of albums are also in the same release group as the original, so shouldn’t it be the same with remix albums (unless of course the remix album is credited to a different artist)?