I am experiencing this problem too and unplugging the monitor long enough for power to cycle off (the LEDs will all turn off, should only be 5-10 seconds, not minutes). This is a band aid solution as you will likely have to do this over and over every other time you turn your computer off. Does seem to be a 50-50 shot on either way if my 1070 strix will detect by display-port on my dual monitors both set to 144 and matched in windows/nvidia. sometimes one port detects and one doesn't but it's a toss up which one does or if they both do or don't. A 5 second re-plug has been my solution so far but I'd like a real one. I am now trying to switch into my power settings into "High Performance" to see if it stops disabling the display-port connections. And yes like most people the DP cables are firmly plugged in, clean, functioning cables. I will report in a few days if the problem has gone away or persists.
