Hey @kongiann
Check out this help article:
https://support.spotify.com/using_spotify/the_basics/listen-offline-troubleshooting/
Downloads unexpectedly removed:
There are a few possible reasons why your downloads might have been removed:
If your tracks won't stay downloaded to your device, we recommend removing all your active offline devices (see "Manage your active offline devices" below), then reinstall the app before downloading the tracks again.
Hey there you, Yeah, you! 😁 Welcome - we're glad you joined the Spotify Community! While you here, let's have a fun game…