Could you try logging out from your account, restarting your device, and logging back in? If that doesn't make any difference, could you let us know if that only happens when Spotify is playing? If so, let us know if it happens when you have an external device connected to the phone, like a Bluetooth speaker or headphones, or if it also happens when playing directly from the phone's speaker.
We'd also like to know if the volume changes when you receive a notification.
Keep us posted.
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