Thanks for this, your suggestion of treating it as a fallback screen was able to resolve the issue.
In case anyone runs into a similar problem, here's precisely how I solved it.
My system consisted of a laptop screen, which will be Screen #1.
Connected via USB+HDMI was an HMTECH 10" touchscreen for raspberry pi, which I picked for its low cost, its compatible with standard windows 10, and since it has no case, I could spread some consumer grade "liquid glass hardener" for strength (across all of our use, although after a few months this screen type has had its touchscreen ability fail (probably digitizer) after many monkey punches, the screen has not yet cracked), and as one more safety layer, we put one more layer of standard plastic screen protector on the surface, so even if it was to crack, glass would be contained until the unit could be swapped for a fresh screen with similar modifications in our customized case that is home cage mountable. This is Screen #2.
When I first purchased the laptop, I chose one that was mid range, that contained AMD graphics, since I had noticed that photodiode tuning required AMD graphics (we do not plan to use photodiode with this system, I just figured AMD might result in fewer driver incompatibilities, an assumption that might not be correct now, as a different lower grade intel system with integrated graphics has been running with no issue since we've created our setup). Previously our AMD laptop had worked "plug and play", screen #2 was selected as the subject screen, and no additional configuration was necessary.
After investigation it appears that a "normal windows update", had updated the AMD graphics driver, and now this "plug and play functionality" has ended. Changing the background color to a different color still shows it as black, although in "extend display mode", all normal functionality on the screen #2 was fine, it was just when the task was started that it shows as blank.
In windows Duplicate and extend settings menu, after setting the screens to
1 | 2
I selected Screen #1 as the subject screen (does not work if screen #2 is selected, even if the other orientation is correct), and set the fallback coordinates to [1920,0,3840,1080] after which point, with "forced use of fallback screen checked", it does now work again.
It is strange, because the operating system definitely recognizes Screen #2 as a separate screen (with resolution 1920x1080), and during normal windows operation, such as moving windows around, dragging programs from one screen to another, all that works just fine, its just when the "low level monkeylogic stimuli appear", and if they are generated on screen #2, it shows an entirely black screen, no matter what the background color is set to. Though, when generated on screen #1, it works.