Asus PB287Q DisplayPort not detecting signal when connected to RX580 via DisplayPort

Tl;dr: check that the monitor is configured for DP1.2.

I'm posting this in case anyone else runs into the same problem. The fix is embarrassingly simple but it took me several hours of searching and experimentation (and many false starts) to stumble across the solution.

Configuration:

• Asus P8Z77-V motherboard, with:
  • i5-3570K.
  • 16GB RAM.
• vanilla install, configured as an iMac13,2.
• multiple monitors (one of which is a PB287Q) connected via DisplayPort to an MSI RX580.
• recently upgraded to Mojave.

Note: I don't think all of the above is important.

Symptoms:

• all displays previously worked connected to an NVIDIA 980.
• the PB287Q display was black; however, the display was detected in MacOS (displays preferences showed the display).
• The monitor displayed a message: signal not detected.
• All screens would flash every 3-5 seconds.
• the PB287Q would work if the displays were mirrored in display preferences.
• the PB287Q would work if configured to use HDMI.
• you could not interact with the PB287Q menu when it was connected to the computer via DisplayPort.

Fix:

• confirm in the monitor settings that it is configured for DP1.2.

Edit: added more symptoms.