Same problem. I'm running Mojave on a Mac Pro 6,1 and I wanted to give Monterey a try. I have two Apple Thunderbolt displays and a Dell monitor running it via HDMI. All three work fine, but when Monterey is installed, one of the Thunderbolt displays will not work, even though it shows up and is marked 'active' when I check the Systems Report. Anyone have any work arounds?