I feel like this has something to do with Apple fucking with DP 1.4 for the ProDisplay XDR.

My 2019 Mac Pro with Catalina could happily drive 2 4K monitors in HDR @ 144 Hz.

People wondered how Apple got the math to work to drive the ProDisplay.

Big Sur? Not any more. 95Hz for 4K SDR, 60Hz for 4K HDR. Not the cables, not the monitors. Indeed, "downgrading" the monitors advertised support to DP 1.2 gave better options, 120Hz SDR, 75Hz HDR.

And it was never fixed, not in Big Sur, Monterey or Ventura, when I had switched monitors.

Hundreds of reports, hundreds of video/monitor combinations.