Am I going nuts! I just went into a very long playlist, one that has numerous ‘unavailable’ tracks in it (as described in the link here Qobuz tracks missing…) and the Playlist Improver found all of them! Most were just lost Qobuz albums that Roon had now found, not reissues, just albums that had changed their ID for whatever reason. WoW! 
Then I just realised my Qobuz albums had all disappeared from my Roon library (as per this link here) and on checking that playlist for unavailable albums none are found anymore… oh well, it looked good for a moment there!! 