I believe your camera is behaving as "normal", meaning it is not compensating your display to see the image knowing that a speedlight will be used. Is it a Nikon speedlight or other brand?
I don't use Manual mode with a flash, so I have not tried that on my Z5. But if you did not have a flash installed on the camera, I would expect ISO 200, f/16, 1/200 in a dim room to look black in the viewfinder. I thought there was a setting for fixing this in the setup menu, but it would take some time for me to research (which I don't really have tonight) this when you could try doing that yourself.