Apple has been hard at work adding more and more bugs, as has been the case for years. Including Thunderbolt 3 bugs, particularly with respect to display. Getting a video signal to my NEC PA271Q still gives me fits on the 2019 Mac mini.
When I returned home from a long trip, my 2019 iMac 5K, all items and cabling totally unchanged/undisturbed would not drive the second screen (“no signal”) using the OWC Thunderbolt 3 Dock. Swapping in a 2nd dock, the display would work, but thr 2nd Thunderbolt 3 port on the dock was dead—no matter the cable or device nothing would work. Rebooting did nothing.
Apple keeps breaking things in macOS and this I deem one more example of sloppy software engineering (I don’t think I am unfairly blaming Apple since this kind of stuff regularly happens after Apple software updates, like the most recent security update).
Having to unplug stuff from the wall to make it work is just insane but I’ll take it.
This suggestion came from OWC: remove power from all devices (unplug), wait at least a minute, plug all back in and reboot.
This cured the dead Thunderbolt 3 port and the display works too! All is back to normal.