I just turned on my phone (sony xperia z5) and opened Spotify to find at that all of my downloaded songs were not recognized as downloaded anymore, and the "available offline" tab is unchecked for all of them. The data is still on my SD Card (about 10 GB) and my phone is still recognizing the SD Card just fine. Restarting the app and restarting the phone didn't fix the problem. If I click available offline, it starts downloading the songs again despite the fact that they are clearly still on the SD Card. Is there any known way to fix this other than deleting it off my SD Card and redownloading everything?
Once you started downloading again, you created a new cache in internal storage and when you switched the storage location back to SD card, spotify will have overwritten the old SD cache.
Can you make sure in android application manager that spotify has permission to access storage. The only time I've had this happen was following a recent firmware update but switching back the storage option before playing anything seemed to work.
I just had the same situation. I am using Pixel2 and don't have SD Card. I granted spotify to use storage and restart the phone but nothing changed. Spotify uses 5gb storage and app shows no downloaded album/playlist.