That's completely f'd up. Brings me one step closer to ditching Macs.
Honestly life has been turbulent the last few years and I just haven't had the energy to switch, even though I think everything I do at this point is easily doable in Linux anyway.
My cheesegrater Mac Pro with a beefy graphics card under Catalina, I was happily running 2 4K displays in HDR10, at 144Hz.
First beta of Big Sur, which came out with the Pro Display? I could drive them in HDR10 at 60Hz. 95Hz in SDR.
Was reproducible - by me, by many others.
Turn DSC 1.4 down to 1.2 on my monitors, and it got a bit better, HDR10 @ 75Hz, 120Hz SDR, IIRC.
That remained broken all through Big Sur, despite a huge number of reports.
Remained broken through Monterey too.
I believe it's fixed in Ventura, but I've since changed monitors.
Initially, people were wondering about the Pro Display's bandwidth and how it was doing 6K HDR and other USB connectivity, and it sure as hell seems like they messed with the DSC spec to make it happen, and 'screw other monitor owners'.