If the service is not responding for too long, the app will use a built-in ringtone. If the alarm starts and your phone is locked we are unable to change the playlist or album in your Spotify app. In this case the app will simply play where you last left it.
This is a companion discussion topic for the original entry at https://docs.sleep.urbandroid.org/faqs/alarms_do_not_play_selected_ringtone.html