it'd be useful to see how many times it is a real problem with R, RP, R6 and R5.
It’s an issue for all cameras, as we’re not going to set them to perform autofocus at smaller apertures and risk being unable to focus in the dark. Honestly, I think Chris shouldn’t have mentioned that, because he made it sound like the lens “isn’t compatible” with certain cameras.
Low light autofocus is precisely one of the reasons I’m considering buying a fast prime, for instance.
For example: is it only visible when the aperture is stopped down one or more stops AND the camera is focussing close to the minimum focus distance?
Yes, it should be more noticeable at close focus, your thinking is correct.
For instance, I have the sample files from James Reader’s review and all his photographs are perfectly focused in the eye, including those shot at f/2.8.
I have tried the lens, but I have to be honest: I barely stopped it down, I don’t think I went past f/2, because I was trying to exemplify my use-case scenario, where I’d have the 28-70mm f/2 on the other camera.
I think I can convince myself that, for me, a RF 50mm 1.2 is the way to go using very solid

arguments like "i only own 77mm filters", "I can't afford to buy a lens hood" etc.

A lens hood costs 3 bucks and 67mm filters are cheap

(not an issue to me as I have many sizes, I keep my filters when I sell lenses).
I have pretty much discarded the RF 50mm f/1.2, at this point. I don’t like its rendering due to that low contrast look it has, its autofocus is a bit slow, and I consider it to be too heavy for a secondary lens (which is what it would be to me).
I’ll have to test the 45mm again, and try to replicate the issue.
Other than that, there’s the 50 VCM, that I also tried and loved.
A Canon lens with an aperture of f/1.2 and offered below $500 USD? Can this work out here? Let's check the Canon RF 45mm f/1.2 STM ...
opticallimits.com
Oh boy, that’s gonna be harsh for sure
EDIT: focus shift detected by Optical Limits as well