If none of the above work or you don’t want to restore a backup then you could do the following:
Make a complete copy of the folder that contains the album, somewhere that Roon is not Watching.
Double check that you have made a complete copy (especially if you do not have a backup of your audio files). If in doubt, stop and do not go any further!
Delete the folder containing the album in the Roon Watched folder.
In Roon Settings | Library | Clean Up Library. This will make Roon forget all about the albums.
Copy the album back to where it was in the Roon Watched folder.
Roon will then reimport the albums unmerged.
If you are at all unsure of any of the above steps, please ignore this method as you could end up losing the audio files.