This bug is impervious to logic.
Has been working fine for months. Suddenly stopped yesterday.
Driver says it doesn't need updating.
Have updated every system option in sight.
As a last resort, I uninstalled the secondary HDMI monitor.
Windows 11 now does not detect it.
(It detected it just fine when I first brought up the computer,
lo these many months, so I suspect a bug in the latest Win 11 update.)
HAH! Solved! After doing all of the above, I tried a HARD reboot, instead
of a SOFT one. (Shut the machine down, then push the start button, instead
of selecting "Restart" from the menu. All of the system-update restarts must
have been soft restarts )
The problem might have gone away sooner in the process, but at no
point was a hard restart required or recommended!