Ah, fair.I thought you said you had to use HDMI. I guess then I figured that no matter which DP port on your monitor you used it shouldn't make any difference. You didn't say that there was a variety of DP ports that you could use.
I'll make it more clear then.
My card has 3 display ports and 1 HDMI port, my screens have 1 DP Port and 1 HDMI port. All 3 DP connections from my card give the same issue on all 3 monitors with DP, but not HDMI.
On some screens like mine and apparently Mares123's you can go into the monitor settings and swap the DP slots mode from 1.1 to 1.2, it's not a different slot just toggles what mode the slot uses.