Hi Richard,
Welcome to the forum, and thank you very much for the detailed description!
Originally, the problem was slightly different, as Archicad crashed while having iPad as Sidecar display. I don't really use it very often, but during our test with a Macbook Pro M1, I tested with an iPad Pro and had no issue back then (running macOS Big Sur 11 and Archicad 24 at that time).
Recently, I noticed a similar issue on Archicad 25 (which seems to be the one from your screenshots?), running on Windows 11: if I minimize it, or put it under another active window, upon returning the Work Environment turns white/grey. A quick zooming in/out would refresh the screen, thus I could continue working without any problem.
I haven't seen any similar report, so it's a surprise to know that something happened specifically on iPad's display as Sidecar. I'm wondering if you have similar problem using only the laptop's screen?
Looking forward to hearing from you!
Best regards,
Minh
Minh Nguyen
Technical Support Engineer
GRAPHISOFT