The current version of the Spotify app is completely broken: if downloads are saved to an external SD card, all of them are lost as soon as the phone is rebooted, even if the SD card is not touched.
As soon as the phone finishes booting, a notification appears that Spotify is downloading thousands of files. Upon examination, in Spotify settings the Storage option has changed itself back to Internal, even though the SD card is still there.
At this point you can change the setting back to SD card, but that will DELETE all the downloaded files and start over downloading them from scratch! This is all the more evident because the Spotify app itself reports many gigabytes of used space on the SD card before the switch, and not after.
I'm so angry about this, but mostly at myself. This kind of nonsense is par for the course with Spotify, the only question is why I keep punishing myself by paying for it.