Too many face this problem. All downloaded songs (offline songs in other words) are gone (seems to be removed by Apple Music from their iPhone) after existing for 24 or 36 hours. How to fix?
Look at the following solution to get rid of trouble if Apple Music keeps deleting offline songs from your iPhone:
My icloud music library setting was switched off (Settings > Music > toggle iCloud Music Library switch)
- Sign your Apple Music out, then Login back into Apple Music app in 6 minutes later.
- Settings > Music > Enable iCloud Music Library (merge contents).
- Launch Music App and wait until library is synced.
- Download all your music songs again for offline listening.
Hopefully the above tip would work on everyone encountering the same issue.