A thread I read somewhere else explained that the data transmission protocol on HDMI can compress the video data, losing some detail in very brights and darks, if the bandwidth needed by the display is encountering the limits of the cable. HDMI version 2.1 is when this feature was added. If your display (like mine, Asus PA279) only supports a lower version of HDMI then compression cannot happen.
DVI and DisplayPort do not have any feature to compress video so they will always be accurate.
Now perhaps this compression might only happen on very large displays, 4K or 8K movies with surround sound also in the HDMI cable, but we don't need the HDMI connection to start creating problems itself!
Andy