I kept getting pop ups from Spotify too, annoyed me, ended up disabling everything I could find in settings relating to the app, still got them...I logged out of it, rather than having it open in the background, and am pop up free now...be a bit of a pain if you use it a lot, logging in and out, but you could try it and see if it makes a difference.
There's an option in its settings relating to notifications and it being open in the background, could maybe try rejigging them, see if it helps.
Have you tried logging in through one of the different options it gives you(Samsung, Facebook etc), see if it makes a difference?
Thank you for your reply. I do use it for hours daily so I'd rather not have to log in/out constantly. Ironically since I posted this the notification in question has gone off and not came back on. If/when it comes back I'll have a look through settings options.