I don’t think that Macs support an audio only HDMI connection and also might restrict the resolution. Assuming that it might check to see if the monitor is on and active, have you checked to see if you are also getting a second screen detected on the Mac Mini? And if the TV that is connected to the AVR is turned on and that HDMI IN active on the AVR?.
Which Marantz are you using?
If you google Mac Mini HDMI on the roon forums you will see several threads you might be interested in