It's because of the HDMI connection reduced bandwidth. Those TV's have HDMI 2.0...
You didn't say what video card you have. Passive "converters" from DP to HDMI make things worse. Also, some video cards, with HDMI 1.4, don't support all the combinations.
My PQ65X is for now connected to my Media Center PC that has a GT730 video card. Well, it doesn't cut it anymore, since it is rated only HDMI 1.4. I'll switch to a Quadro P620 soon, I am waiting for my active miniDP-HDMI adapter to arrive (rated 4K 60Hz).
My PC monitor (AOC U2879VF) is connected with a DP 1.2 cable to the GTX1080 and I have available 4K RGB 10 Bit Full.
Last edited by SoNic67; 11-24-2019 at 11:39 AM.