Hi,
Sync Library is most likely the primary cause; if it is disabled or not syncing correctly, changes made on one device are overridden by the cloud library, causing added songs to vanish from collaborative playlists.
To fix this, ensure Sync Library is turned on in Settings > Music on all devices, then toggle it off and back on to force a resync.
If the issue persists, check the following:
- Cloud Verification: Visit music.apple.com on a browser to see if the songs are in the cloud library; if they are there but not on your device, the local cache may be corrupted.
- Third-Party Apps: Using third-party apps to add songs without Sync Library enabled will result in data loss, as the changes are not reflected in the official cloud library.
- Optimize Storage: If "Optimize Storage" is enabled, downloaded songs may be removed to free up space; ensure this is set to None or check Settings > Music > Downloaded Music to re-download missing tracks.
- App Glitches: Force restart the Apple Music app or your iPhone, and ensure your iOS and Apple Music app are updated to the latest versions, as bugs in older iOS versions (like 17.3) can disrupt collaborative features.
- Rejoin Playlist: If a specific collaborative playlist is malfunctioning, try leaving the playlist and rejoining via the invitation link to reset the connection.