U2717D, color with low RGB values are truncated to black

Bought a U2717D recently, but I find it cannot display dark colors. For example, when RGB channel values are less than 10, colors like: RGB (10,10,10), (9,9,9), (8,8,8) are all displayed as total black (0,0,0), I cannot see these colors from the black background (0,0,0). It seems like low end of the gray scale is truncated to full black (0,0,0). Do you know where could be the source of the problem? My computer problem or monitor firmware bug? I do connect it to three of my computers with Windows 7 and Windows 10. All see the same problem.

Computer configuration used =
GPU: Intel HD 4600
PC mDP out to monitor DP in
OS: Windows 7 

From the image below you can see there is text OK in the middle, but it appears as total black on my screen:

text ok with color rgb:(10,10,10)