@eamon and All,
So I worked with Eamon today via Team Viewer to address his issue and we got everything working. Mostly, the issue seemed be related to the fact he had scaled the two 4K displays (LG Televisions actually) to 50% using the Displays System Preference. (Eamon's instinct here was a good one: I don't have 4K content, so let's save some bandwidth.)
But, there were several other anomy'salies that were in macOS itself, e.g., the System Profiler reported the resolution of one of the displays to be 7680 x 4320, even that would be an 8K display not a 4K one; this finding was especially strange since two two displays were exactly the same make and model, and certainly do not offer an 8K resolution.
Our action item will be to double-check that using the scaling to something other than 100% will cause Isadora to do something wrong. But right now, I do not feel this is some widespread issue that others will face, especially Ince most users would not
Thanks to Eamon for getting me access to his machine and for standing by as I went through this process remotely.