Update. Sorry, spoke too soon.
I realised on closer examination that the only tracks showing in the Devices/iPhone/Music section of the iTunes sidebar were those from the one playlist that was already synced to the phone via its Finder entry (ie, from the Apple Music library). In iTunes there were no Artist/Album/Genres/Playlists tabs to tick or untick.
So I tried quitting iTunes and unticking that playlist in the Music section of the phone's Finder sidebar and applying the change. Then I restarted and launched iTunes again. Now there are no items in Devices/iPhone/Music.
This appears to indicate that Apple Music retains control of the iPhone's music content regardless of what's going on with iTunes.
I realised on closer examination that the only tracks showing in the Devices/iPhone/Music section of the iTunes sidebar were those from the one playlist that was already synced to the phone via its Finder entry (ie, from the Apple Music library). In iTunes there were no Artist/Album/Genres/Playlists tabs to tick or untick.
So I tried quitting iTunes and unticking that playlist in the Music section of the phone's Finder sidebar and applying the change. Then I restarted and launched iTunes again. Now there are no items in Devices/iPhone/Music.
This appears to indicate that Apple Music retains control of the iPhone's music content regardless of what's going on with iTunes.