Thanks for the link TotoyZ4, I assume I can't see the graph since I don't have an account at blu-ray.com. I would have to say my experience is the opposite of yours in HDR, I find the bars seem not as black during darker scenes, but "appear" very black during a high brightness/contrast outdoor scene. I know they are not same set but there is alot of HDR discussion in the Z9D thread and depending on picture mode they use contrast from 86-91.
I don't know if anyone else here plays Battlefield 1 but for reference the game has an HDR adjustment slider that goes from 0-10000 to match the brightness of 2 white boxes. When I used Backlight - Max and Contrast - 95, I arrived at a value of 2600. Later I turned my Backlight setting to 48 (didn't seem to make a difference) and turned Contrast down to 90 and had to pump the slider up to 3600 to get the same result. Same type of adjuster in Horizon Zero Dawn, and I had to increase the game brightness setting from 30% to 40%. If nothing else these two games are a real treat in HDR on this TV on the PS4 Pro.
I also have Advanced Contrast Enhancer set to Medium, I have yet to notice any annoying brightness fluctuation like this setting caused on my 850C with frame dimming, obviously it must work differently with FALD.
Sony XBR75X940C, UBP-X800, PS4 Pro 5.1.2 System Yamaha RX-V1081 Paradigm Studio 60 V5, Monitor Center 3 V7, Titan V2 Klipsch RP-140SA DIY 15" Sub w/Inuke1000DSP