Steps to reproduce:
1. Open Spotify app on an iPhone
2. Start playback on a Raumfeld speaker with Spotify Connect
3. Exit (background) the Spotify app
4. Push volume up/down buttons or rotate the hardware knob on a Raumfeld speaker
Nothing should happen
iPhone device's volume is changed
Happens since the introduction of now playing being visible and controllable on lock screen for Spotify Connect. Before the lock screen feature was only available for local (phone) playback.
Reproducible with iPhone SE (iOS 10.2) and iPhone 6s Plus (iOS 10.1), Spotify app version 188.8.131.524, with latest Raumfeld firmware (1.61).