Thanks, but just to be clear, I don't see any problem in real content or on a solid gray screen of any luminosity.; the latter are correctly tinted with no hint of the purple semicircles at the bottom. It's only the 1x1 pixel checkerboard that gives results as depicted in my photo. I do see a tiny hint of it with a 1x1 grid. I see none of it with a 2x2 pixel checkerboard. It is... Strange.
I wish I hadn't stumbled upon the 1x1 checkerboard pattern. I think it's just a pathological case that will never be a problem unless I do something weird like run an old OS that uses a simulated gray background in a VM on the TV. I'd still be interested to hear what others see when displaying the pattern, which is in AVS709HD as I previously described.