Music syncing only purchases, not full library, to iPhone

Update: I have created a ticket with Apple Support, Case ID: 102564756894. So far I've spoken to 6 different tech rep's (so far), 3 of them senior, none of whom have been able to diagnose the problem. We can only confirm that the problem with syncing music to my device happens with iTunes and with Apple Music/Devices, and that both applications are referencing the correct library file locations for each song.

By chance I noticed that the songs which are not syncing are mostly MP3's, downloaded from many other sources such at 7-Digital, SoundCloud, Bandcamp, or directly from an artist's website. These songs were all organized correctly in iTunes before, complete with album artwork and details. The music library continues to organize these songs into their respective playlist correctly, but the songs themselves do not play from within the Apple software. There is no problem with song playback if I open the file directly from Windows Explorer/Finder, and play using VLC player or Windows Media Player; so we can confirm that the song file is not corrupt in any way. Here's one example:

The bottom song is the original MP3 as the library playlist has saved it, located in the correct file folder. The top song I simply drag-dropped the same exact MP3 into the playlist window again, and now it shows all the album artwork/details again. It also moved the file location from 'C:\Users\_\Music\Apple Music\Media' to 'C:\Users\_\Music\Apple Music\Media\Music' automatically, despite me not having that setting selected.

So new problem hypothesis: the songs which are not syncing to iPhone are all similar to the above example; they cannot be played in any Apple software until the exact file is re-imported into the library.

Obviously I do not want to go song by song through +6000 files to re-import into a library, which already appears to be organized just fine. Because these songs had no playback or syncing problem in previous software versions, this seems like an obvious bug Apple needs to address. Barring that long process of software development, what can I do now to make these songs read correctly (en masse)?

Just for good measure, I repaired the security permissions as you suggested. I also re-installed iTunes directly from Apple (which, FYI, contradicts Apple's own tech support recommendations to download from MS-store or use Apple Music instead). iTunes works better than Apple Music so I'm happy to revert back.

You are correct, the songs will still not play within iTunes, despite the library (seemingly) being organized correctly. The same issue exists where file locations are 'found' correctly from within iTunes, confirmed by selecting any file and "showing in windows explorer". A song file location matches the parent library folder location as defined by iTunes' settings: C:\Users\_\Music\iTunes\iTunes Media\Music\[artist folder]...

I do not use OneDrive or any other cloud drive service; all music files are saved locally per the default folder locations.

I would be interested to try a script to fix broken links, if you think this is most likely the issue. I also would be interested in repairing the ID3 tags as suggested in your earlier link, but iTunes has that option greyed-out for me, presumably because it's still confused about file location.

I agree, iTunes was a far easier experience to work with than Apple Music, but it was a forced upgrade from Apple so iTunes no longer works to transfer music files to devices (at least using Windows OS).

The issue still persists.

Unfortunately uninstalling Apple Music+Apple Devices did not work. Opening iTunes after that process and connecting my phone only showed a message saying that downloading the new Apple software was necessary before continuing.

I also tried some different sync settings within Apple Devices which did not work. Removing all theoretical restrictions I opted to sync my "entire (56Gb) music library" to the phone so there should not be any reason why one playlist or individual song would be left out. Still however, only 5.4Gb of audio is being transferred over.

Not sure what I should try next here, because I can confirm Apple Music contains the correct song content I want to transfer, and Apple Devices sync settings are seemingly selected properly to transfer my entire library, yet it still does not.