Playlist entry creating duplicate album and track entry

So, for example, Album 'The Doors' contains a track called 'Soul Kitchen' - listed correctly in LMS
Creating a playlist that includes 'Soul Kitchen' results in no change to original album listing, but (at some point) does create an additional instance of 'The Doors' album, containing a single track of 'Soul Kitchen'.

Have had several trawls to see if somebody else already had the answer so apologies if it is there somewhere, but the closest I have found is the suggestion that this is caused by having the Playlist folder within the Music folder (and hence creating duplicates), which is not the case here.

My Library is on a Synology NAS. I maintain the original library on my PC, it backs up to the NAS, and that is the directory that LMS uses.

Media folder: /volume1/music/XPS2/J/Users/XXX/Music
Playlists folder: /volume1/music/XPS/C/Users/XXX/Music/Playlists

You can see the file paths are different, and in fact clearing the library and rescanning from scratch resolves the problem - no more additional albums, so it would not appear to be the location per se.

Maybe the problem only occurs when the Playlist is actually played? Is it a play count thing?

Can anyone explain what is happening and how to stop it?