This is weird… as I am no longer experiencing the blank/black screen on lock.
Now that I’ve accepted the monitor’s DP demise and swapped connecting the monitor via its still working HDMI port, I cannot duplicate the blank/black SDDM on lock problem.
However, curiosity demanded I see what happens when I disconnect the monitor (unplug HDMI) and my next lock after doing so presented the blank/black screen.
Reconnected the HDMI connection, desktop spanned two monitors, locked the PC, and SDDM showed correctly; no more blank/black screen.
This got me even more curious, wondering if kscreen was holding on to the fact that there were 2 monitors, so I tried another test… disconnected HDMI, rm -rd ~/.local/share/kscreen, locked the PC, and SDDM was blank again.
Well, I’m confused, but it appears that a workaround to this issue under X11 is to have more than one monitor.