I also have this issue. I intentionally got a 4k external monitor that I am able to run off one USB-C cable to simplify my mobile setup. It was working flawlessly with Big Sur for the past year, taking advantage of 2-way power delivery and DisplayPort over USB-C with USB-C 3.1 Gen 2 on one cable. Please note, this monitor is specifically DisplayPort over USB-C and not Lightning. Now on Monterey, the monitor gets detected as an audio device, but no signal is displayed (black screen & not in Display settings), no sound is played and moreover the OS gets laggy. It's apparent that the Macbook Pro is still able to supply power, but something is wrong with the display signal that is being sent. Moreover, the monitor still works perfect on any other device that I connect to it. I've heard in some other places that the fundamental problem is that the DisplayPort 1.4 protocol is not implemented properly in Monterey and that you could circumvent the issue by changing your monitors settings to use DisplayPort 1.2, but my monitor does not have options to change the DisplayPort version. This is a hugeeeee hinderance to my workflow so I would really appreciate an update on Apple's end.