I know it sounds simple, but it's often overlooked. Could you try restarting your device? Sorry if this may sound a bit repetitive but if that doesn't help, try a quick clean reinstall of the app by following these steps.
However, you're on iOS 12.1 Beta which is an unreleased versions of iOS, currently the official version for all the users is iOS 12. You might also be interested in signing up for Spotify Beta for iOS 🙂
If you're still having trouble, could you let me know the exact version of Spotify and the exact version of the operating system you're running?
I'll see what I can suggest next!
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