I saw another thread here
https://forum.blackmagicdesign.com/viewtopic.php?f=3&t=84129 asking essentially the same question: how can I send a 4K signal to my 4K monitor (I'm on a MBP 2018 with DeckLink in a Sonnet ePCI box)...the BM Desktop Video Setup app is set to 2160p30 in the Video Output's "General" section, but the info for that setting says "Final Cut Pro X and Startup Video Output will use this setting." What does that mean, and why wouldn't Resolve also use this setting? Resolve appears to use the same setting as the SDI Output section in the Video Output tab.
What might be going on, and how do I get it to output at UHD?