Music playlist syncing problems - tracks from Apple Music subscription and images

I have come across a peculiar issue with the sync between devices using Sync Library. I first noticed it when updating images to various playlists on iOS on iPhone and adding one track from Apple Music to one of those playlists. When returning to my iMac, none of the changes had been made to the corresponding playlists on the Music app there. What's worse, when I then went through the various sync issue troubleshooting steps (restart devices, restart apps, stop sync and resync, log out of account and log back in again, update both to v26.1...), the images that I had added to Music on iOS were suddenly all removed and replaced with the standard 'quadrant tile' (from playlists created years ago) that consists of thumbnails of the first four media in the playlist being placed in quadrants. Furthermore, this wasn't replicated on the Mac - the images there were now just the blank pair of quavers image for each playlist (presumably indicating it used to have an image but now can't find it). When I then tried to add images to the Mac version, when saved they persisted until I then went to the iOS version of the playlist to check if had synced (which they hadn't - it was still the quadrant image) and then they suddenly disappeared again from the Mac version again on opening the playlist in iOS! This repeated twice more but on the third occasion of saving the image I managed to get it to persist in the Mac version. They are still not syncing with the iOS app though which still show the quadrant images. One step forward, two steps back...

Next, I had also added another track selected from Apple Music, rather than my personal library, and this also didn't appear on the Mac Music app. Checking the track count, I discovered that in addition to this newly added one, another track was missing from the Mac app. This also turned out to be a track from the Apple Music subscription that I didn't own myself. My subscription remains up to date and I can otherwise play all these tracks. What was even more peculiar was that when I tried to add these two tracks into the playlist on the Mac, I got the error message telling me I was attempting to add duplicates, despite these not showing. If you select 'add duplicate' they still don't appear BUT are then duplicated on the iOS version! When playing the playlist and skipping past each playlist track on the Mac app, the missing tracks don't play either. The Mac app seems to know they are there (and in the sync) but just won't display them.

HOWEVER, this is not consistent. I thought I'd test out other playlists and tried doing the same thing with another one - adding a random track from Apple Music subscription on iOS and see if it synced through to the Mac app. It worked fine, inexplicably. I then found another playlist that had a mismatch of track count between the one on iOS and the the Mac one, identified the offending track from Apple Music and tried to add it back in to the Mac app. Again, same duplication message - so it's there, but it's not there.

I don't think it's too much to ask to just be able to make changes on one device and expect to see that flow through to another and them to match perfectly. Or is it? I really cannot see that I am doing anything particularly wrong. Any advice, and errors pointed out, gratefully received.