It may be just that you are logged in / paused on the Spotify app yourself, on your account, even if no music is playing.  Make sure you are out of your own account on all devices - including the phone your Sonos app is on.  (The Sonos app is just a remote control, not a music player.  The ‘device’ as far as Spotify is concerned is the speaker.)

Check which account Sonos is using, the UX for selecting multiple accounts on the same service has historically been poor. Make sure your Sonos controller is using a different Spotify account to the one used for direct playback.

Or just use Sonos for both accounts and avoid direct phone playback: A Sonos Household counts as a single ‘device’ to Spotify in this regard.