Incorrect UPnP notification when volume muted

Volumio Development Bug reports
1

Prerequisites

Volumio Information

Volumio Version: 2.799 on PC and 2.806 on RPi
Hardware: VirtualBox on PC / RPi4
DAC: ALLO BOSS on RPi

Steps to Reproduce

  1. Subscribe to UPnP RenderingControl events
  2. Change Volumio volume using the UI
  3. Display received UPnP notification - this correctly reports the new volume setting
  4. Mute the sound using the Volumio UI
  5. Display received UPnP notification - this incorrectly reports that the volume has changed to 1, instead of reporting that the volume is now muted.

Additional Information

Below are the notifications received in steps 3 and 5:

After changing volume to 22, the following (correct) NOTIFY message is received by the subscriber to the Volumio UPnP RenderingControl service events:

NOTIFY / HTTP/1.1
HOST: 192.168.2.155:8824  
CONTENT-TYPE: text/xml; charset="utf-8"
CONTENT-LENGTH: 338
NT: upnp:event
NTS: upnp:propchange
SID: uuid:0b92f3bc-ef44-11ea-a6d9-bd42d040d167
SEQ: 4

<e:propertyset xmlns:e="urn:schemas-upnp-org:event-1-0">
<e:property>
  <LastChange>
   <Event xmlns="urn:schemas-upnp-org:metadata-1-0/AVT_RCS">
     <InstanceID val="0">
        <Volume channel="Master" val="22"/>
     </InstanceID>
  </Event>
 </LastChange>
</e:property>
</e:propertyset>`

After muting the volume, the following (incorrect) NOTIFY message is received:

NOTIFY / HTTP/1.1
HOST: 192.168.2.155:8824
CONTENT-TYPE: text/xml; charset="utf-8"
CONTENT-LENGTH: 337
NT: upnp:event
NTS: upnp:propchange
SID: uuid:0b92f3bc-ef44-11ea-a6d9-bd42d040d167
SEQ: 5

<e:propertyset xmlns:e="urn:schemas-upnp-org:event-1-0">
<e:property>
    <LastChange>
        <Event xmlns="urn:schemas-upnp-org:metadata-1-0/AVT_RCS">
        <InstanceID val="0">
            <Volume channel="Master" val="1"/>
        </InstanceID>
        </Event>
    </LastChange>
</e:property>
</e:propertyset>

Instead, one would expect <Mute channel="Master" val="1"/>.

PS. The volume mixer is set to HARDWARE. Also, exactly the same behaviour can be observed when controlling Volumio using BubbleUPnP - clicking the speaker icon in Bubble simply decreases the volume to 1 instead of muting it.