Ok so I found the solution, in case anyone else finds this thread via search. To find the (very long) path that Roon thinks your music is in do this: View the track file info and then click “copy file path to clipboard”. Then do a search/replace on the Plex m3u8 file so that the paths match. Put the new m3u8 file into your Roon music folder structure and do a re-scan. Bingo! It should appear.