I leave this here in the hopes it might help someone else in the future...
I managed to fix it myself for the display I was using. I had to make sure the input source on the display was actually called "PC" and the make sure the the "HDMI UHD Color" menu option was enabled.
Not sure why the builtin laptop graphics HDMI output didn't have these issues, nor why a 2nd borrowed display from a different manufacturer also showed the same problem. If there was something that could have been done in the display driver to just respect the reported EDID timings (which I assume the builtin graphics did) then this might have just worked.
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