I've tested this on multiple different installs, all Apple Silicon though (either M4 Mac mini, or M1 Max Mac Studio) in every case, changing the Audio Master has no impact on the actual output volume no matter what output source is set in either MadMapper or OS level.
Changing the volume on the media file itself (standard .mp4 video) does work - but in this particular case, we are using the Link Audio Level to surface opacity as part of switching videos on the fly on a trigger from an external source - but also want to adjust the volume level on the fly
one of our team had a 5.3.4 demo version installed and the audio master level control is working on that one - so I'm going to go back through earlier versions I have downloaded to see if I can find any that do work to see where the bug maybe showed up - but it's kind of a pain to do this with the licensing