I've managed to nullify the drain by force closing the app after every use (as well as removing the Watch app, just in case), so something is keeping it awake. Connect devices would make sense but I'm not siging out of all my Google Homes/Sky Q boxes, It makes the Idea of Spotify next to useless if so.
As per screenshot, it cant be coincidence that my Play Services up time is the same as Spotify. It seems Spotify is using play services whilst in use (pinging something, API call, whatever) but NOT terminating the Play Services process in the background, keeping itself awake.
You can also see the 2 minutes I forgot to close the app down...
It seems to me it *MAY* be a permissions issue. Can those with issues please go to Settings > Apps> Spotify > Permissions and confirm, IF you are having background Drain that NO permissions have been allowed? Ie Camera, Storage, Microphone and Contacts. I'm Guessing permissions are OFF by default. If you could grant access to all these and go back into Spotify, Play a track, Pause playback, Exit the App but NOT close it down or force stop it (Swipe away the notification bar widget) and then check on background usage 5 Mins later and tell me if there has been any activity.
So far I have been able to stop ALL background drain by doing this and can see it happening again if I revoke the Permissions. It seems that on Installation, there were NO prompts to allow access to the above and it is polling Play Services on a constant basis, in a massive loop