Plan: Premium

Country: UK

Device: Samsung S21 (syncing with Windows 10 PC)

Operating System: Android 11 (syncing to Windows 10 PC) 

My Question or Issue:

After syncing/downloading local tracks (from a desktop PC to phone via WiFi) to either a playlist or Liked Songs the tracks become unplayable with the listing greyed out and the green tick disappears.

They work fine at first (when on WiFi and with the desktop on the same network) but once I've taken the phone out and it's either offline or just connected to just mobile data the tracks become unplayable. (see images of just after syncing vs. later on using mobile data).

The local tracks will play on the phone when on WiFi and with the desktop on the same network (sometimes they still appear greyed out and/or pop up with the "can't play this right now" message but will still play after pressing them again). The main issue is when they become later completely unplayable.

Weirdly tracks that I synced months/years ago appear to be unaffected. It's only new tracks I'm trying to add now and those within the last couple of months or so.

I've tried many suggestions online (none have worked) including:
Toggled explicit songs off and on
Repeating with different/new playlists
Local files off and on
Re-syncing/downloading
Clearing app cache
Reinstalling app and re-syncing
Checking file codecs are all normal

The files do sometimes seem to appear and be playable via the in the local files section in the mobile app. Also, if I go in the file browser on the phone it seems to be constantly the renaming files and also generating copies of the local files with "(2)", "(3)" at the end of the filenames of the duplicates.

If I go local files section in the mobile app and try to add the song from there to a playlist it then will not be playable on the desktop (so the same problem but the other way around).