Spotify changes external storage to internal storage at each reboot, deleting all songs and having to redownload everything. The reason is that Samsung phones need about 30 seconds after boot to mount encrypted MicroSD card. The Spotify BACKGROUND service starts before (automatically, without user interaction) and so does not find the MicroSD card. It then switches to internal storage without user interaction and de-syncronize each playlists.
A simple workaround would be for the service to wait 60 seconds or until the app is launched after boot time before checking the storage state.
The issue is apparently present since at least 2016 on all Samsung phones including the S8/S8+ and Note 8. Without resolution, I am thinking about switching to a concurrent service.
I understand your frustration. Great to hear things are working again after the solution @charno posted. Rest assured, the right folks are still looking into this. If you need any help in the meanwhile, feel free to give me a shout! :)