Recently I’m finding several problems with the Spotify app iOS version. For example once my whole offline music got deleted without any reason and I got kicked out and had to sign in again and redownload everything! Or lately some tracks have to get downloaded again! The green offline mark on my playlists gets grey and I scrolled down to the end and saw some tracks are not available and I connected to the internet to let them get downloaded again!
One thing which might cause the problem of your music being deleted or undownloaded is if someone else is sharing your account. You can go to your "account overview" on Spotify.com and at the bottom of the page there is an option to "sign out everywhere". This would remove your account from someone else's device. Then try signing back in on your device.
There could be other reasons your Spotify is glitching but I hope this helps 🙂