As in title, I had this problem on a previous phone, it was damaged so I blamed this on my phone but I'm on my new phone and today I find my "download" button unticked, and my whole music deleted from my sd card. It now happened more than 10 times, first time on a new phone. Is it just me or does this happen to many people?
It seems that your Spotify's cache files might be duplicated. This happens for various reasons: a cache-clearing app might have deleted the original Spotify's folder or your phone's battery died while using Spotify, for example. When the app resumes the download, it creates a new cache file.
Let's try removing all of your offline devices here. This will delete also de cache file that was duplicated.
After that, just reinstall the app following these steps. Finally, you'll need to download the music again.