ODROID C2 Spotify Connect Volume Control

I use volumio on an Odroid C2 which is connected via USB to a Denon PMA-50 amplifier. I disabled volume control (neither software nor hardware) in the volumio settings, but while using the Spotify connect plugin I get to adjust the volume from my smartphone (from within the Spotify app). This does not work on a Raspberry Pi 3 using the same settings. Does anyone know what’s going on here? Any thoughts are greatly appreciated.