Ok, problem solved on my 50 C-Sonnar, just put a little black silicone over the offending screw and no more problems even in direct sunlight.
I also made an interesting discovery. Curious as to why my other two ZM's don't have this problem while my 25 and 50 do, I had a closer look at their flanges.
On my 18mm Distagon, it turns out I have the older bayonet which is not as easy to code, so that answers that one.
However, on my 35mm C-Biogon, which has the newer flange, it appears that the countersinking for the screws is MUCH more minimal than on the 50mm C-Sonnar. I think Zeiss know about the issue, or have a new QC issue they weren't aware of.