First question for the users would be, does the Mac have an actual HDMI port or are they using dongles?

Most of the time when we hear Mac problems, it has to do with the dongle they are using because they bought a version on Amazon instead of an actual Apple approved dongle.

Other times, it's typically an EDID problem and being that your routing video through an AVR, it's not too surprising to me.

Also, you shouldn't have to push back. Your proof is that you plug your PC in and it works.