IMO opinion, what you're asking is as much a hardware question as a software question ( NLE). I had recent issues with PC clone editing computer and a mini monitor card ( pci-e ) using SDI out, to a field monitor ( Lilliput ? ), using old PPro version ( CS6).
It got really complicated.
I just upgraded graphics card, OS, and installed Resolve 15, AFTER just pulling out the mini monitor card.
Like, in a few days, I did that. I have NO use for the SDI out ( cause I also got an Eizo monitor for primary at rec 709 emulation).
Frankly, to put that card back into the computer would require someone PAY me to do it.
I can use the field monitor with HDMI pass through if I shoot to want that, just using camera and recording stuff while shooting.
The cost of the card was not worth the problems with sloppy code from Adobe and lack of standards between manufacturers ( adobe, black magic, apple - hardware AND OS stuff ) for me personally. Black Magic had new options in PPro to start projects ( under Black Magic presets ) and there were options for video ( cuda, OpenCL, etc. ) and also settings for audio, and that is just too much for my pea brain to deal with.
You do have a monitor connected through the BM card, correct?
How about giving the details of your monitors and their connections to the computer?