I just lost a long and detailed piece on trouble shooting this, so be happy you didn't have to suffer through it...
There are a lot of variables here, and I'd suggest you try eliminating a few first. So take out one graphics card and connect only one display, I suggest the TV. Then test for function and if the problem doesn't show then add the Cintiq and see is that breaks things. Last step would be to add the second graphics card, and if that breaks things then swap them over in the slots and see if the problem follows the card, the display, or the slot.
If you get the problem directly with only once graphics card and the TV then you might have a color space mismatch between the computer and the TV. First I suggest you explore the TV menus and look for anything that has to do with color space, white or black point or something like that. I just checked a Samsung TV and I was able to select HDMI UHD Color and HDMI Black for each input. I'm guessing (as Samsung didn't feel like using the standard names) that UHD Color selects between BT709 for Off and BT2020 for On, and that HDMI Black is used to select between a Black Point of 16 and White Point of 235 as in the ITU BT601 standard (same in BT709 and BT2020?) and a Black Point at 0 and White Point at 255 that the PC usually uses.
If the BP and WP doesn't match up in source and display then the picture will either be washed out or over saturated with murky black shadows with no details and burned out white areas. The TV I looked at had the HDMI Black set to Auto, which seems to work for me, but possibly could have a problem detecting the correct setting, so I would definitely suggest you try playing with that if your TV has a similar option.