Volume control for mpd client

I am running Volumio 3 on Raspberry Pi with HiFiBerry DAC+ Pro, and all is fine using Volumio Android application, including volume control. “Mixer type” is set to Hardware (recommended as best setting).

Now, if I am use for example MPDroid android application, volume control does not work. Activating parameter “Volume control by MPD clients” has no effect.
On my desktop (Debian Gnome), using for example gmpc, the volume control is marked as deactivated.

Is that expected ? Is it related to the “Mixer type” setting ?

I am fine using Volumio App, but just to have a better understanding of the situation, if someone can explain ?
Thank you !