I'm running Avid Media Composer 2021.05.0 and I installed the BlackMagic Desktop Video 12.3 driver after I installed Avid. I got the strange issue Avid only regonizes my DeckLink Mini Recorder 4K and not my Decklink Mini Monitor 4K that I'm trying to use. (Both are PCIe cards within my 2019 Mac Pro)

The HW/SW settings only shows the recorder, while my Desktop Video software shows both. (And the monitor also works fine within DaVinci Resolve) I made sure to test when rebooted without opening Resolve, to make sure Resolve is not getting in the way of Avid. 

Has anyone come across this issue before? Any ideas on how to fix this issue are really welcome. 

Support for multiple Hardware devices is a very new MC feature. So with an older version you will only see the hardware MC grabs first.