I have same BM 8K pro and have set it up in UE 5.5 off the Launcher. All connects as expected. My hunch is it is something to do with the way the card is setup in the BM Desktop Video Setup. It is a bit fiddly. Double check and adjust any settings. I suggest you run BM media express to verify the feed coming in on the pin expected. Once you see video take a note of which pin and clear out your media bundle setup and re add. I believe somehow that your decklink card is in a state where all four pins are sending out and there is not a pin with any video coming in.