Toggling Monitor Fading on seems to disable RTX HDR in certain (all?) games/fullscreen applications. Tested in borderless windowed & fullscreen (with and without Windows' "Disable fullscreen optimizations" toggled).

A niche problem, I'm sure - but I really love the Monitor Fading feature of DisplayFusion while gaming. But not as much as I like HDR Not sure if a fix is feasable or not. Seems like a solution on your end would be difficult.