I put the Mac Pro, the keyboard, the new HDMI cable that I bought, and myself into the car and headed to the local "Mac Doctor". About a 5 mile drive over two sets of railroad tracks.
The Mac Doctor plugged in his cable to an available display, plugged in the power, pushed on the button, and voilà, there was Catalina Island.
He speculated that either: 1.) the trip had broken through corrosion in contacts (which seems reasonable), or 2.) Due to several USB and other connections, a circuit had been formed that stopped the display process (not all that unreasonable).
He suggested, and I concurred, that cleaning contacts was in order, and that, when I got home, I plug in only the display and the computer to see if it still worked.
It did, so I continued to add items. So far, no failure. Tomorrow, 99.9% Isopropyl alcohol and cotton swabs.
I have heard that there is (was) a way to flash the rom on the PC version of the RX 580, however, like you said, it would need a larger firmware chip to hold both sets of instructions (which this special version has).
Thanks for confirming what I thought was the case.