This has happened on my Forerunner 645/M too.
No fix, I just hard reset the device and loaded everything back. There is still some wonkiness.
Next time I'll try the easier 'delete and reload the app'
It has been awhile. I think what worked for me was going into the watch, then into the spotify setting, then logging out on the watch. To log back in you have to have your phone, and it reasks permission to log back in. kind of a pain. But it seems to have worked for me.
Okay, will try next time. I deleted Spotify and reloaded it.