I can replicate your situation with my MacBook Pro, core i9. I’m outputting HDMI from the MacBook Pro to an older NAD 765 home theatre. I can only get the NAD to play all 7.1 channels (with associated NEO 6 and EARS options on the NAD 765), if I set the MacBook Pro MIDI options to 2 channels. But in that situation, then I get a lot of drop outs.
If I set MIDI to 8.1 on the MacBook Pro, I get only the left and right speakers to play. No surround side or rear surround. Only options on the NAD are Direct and Stereo Downmix. No drop-outs at all.
But seems like I should be able to come up with a solution to get all 7.1 speakers to play with no drop outs.