@Robert_Daines I think you have run in a well known Roon limitation regarding composition (work) identification. Roon does not allow you to create a composition, only link to a composition which is already in its database (coming mostly from allmusic). In the case of your Sibelius album (https://www.allmusic.com/album/sibelius-the-complete-symphonies-mw0001821319) allmusic identifies the 7 tracks of disc 3 as belonging to the same composition Symphony No. 5 in E flat major, Op. 82. Allmusic does not know about the two different 1915 and 1919 versions of this symphony, just about the usual Op.82 version. So does Roon, since Roon does not curate its metadata and gets all metadata from its providers.
Unfortunately, Roon does not allow creating new compositions. So when you tag tracks 1-4 with “Work=Symphony #5 in E flat (Original 1915 Version), Op. 82”, Roon does not create a new composition by this name (as you hoped it would do!). Rather Roon does its best to find a match with the provider supplied metadata. Using the composer name (Jean Sibelius) and the opus number (Op.82), the match is with “Symphony No. 5 in E flat major, Op. 82” as found on Allmusic, and this is the composition id displayed on the album page. Same reasoning applies for tracks 6-7. Setting the WORK tag on those tells Roon to group those tracks under a new composition (instead of lumping those in a unique 7 part work as on the Allmusic album page). But the match is still to the single composition entry for Op.82 available on Allmusic, and this is what appears on the Roon album page.
Given the way Roon obtains the composition metadata (no curating, all coming from the metadata provider), I feel the only way to get distinct composition ids for Sibelius Op.82 would be to contact Allmusic and ask them to create alternate composition entries in their database and assign the new metadata to the Vänskä album. Will they do it, and how long will it take ,that is anybody guess …
Or you could press Roon to finally come up with a way to let users define/create compositions. This has been asked many times in the past.
I have personnally run into this problem many times.The best workaround I found is to include in the PART tag the intended composition name as a prefix of the movement name . This can make the PART name somewhat long, but since Roon will always show the exact wording used in the PART tag in its displays, a least you get an unambiguous clue of what composition version you deal with. In your case for example, the WORK tags for tracks 5-7 would become:
Track #5: Symphony #5 In E flat (Final 1919 Version), Op. 82 1. Tempo Molto Moderato
Track #6: Symphony #5 In E flat (Final 1919 Version), Op. 82 2. Andante Mosso, Quasi Allegretto
Track #7: Symphony #5 In E flat (Final 1919 Version), Op. 82 3. Allegro Molto
Hope this helps