All sarcasm aside... does anyone not see how this could be a serious issue for astrophotographers?
... it just doesn’t appear there is an issue to me.
Wrong. It's a HUGE issue for anyone who's favorite photographic subject is the inside of a lens cap!!!
If you are using a headlamp or the LCD backlight to set your exposure for a night sky capture, you could be in serious trouble. I'll be doing some tests tonight and will follow-up tomorrow.
I fear the 5D3 may not be suited for astrophotography at all if this problem affects nighttime exposures.
Because you would NOT rely on the camera's idea of what is correct exposure when doing astrophotography. Because if you did, the camera would seek to make the picture's lightness similar to that for normal photography.
You simply don't use the camera's exposure system when photographing star fields, because the overwhelming majority of the field is near-black.
You go to manual, or bulb.
Unless you're doing high-magnification photos of the Moon or other frame-filling objects, in which case you're now doing daylight photography, and basically, the terrestrial sunlit "Sunny f/16" rule applies.
Also, you're unlikely to be using headlights or bright torches anyway when engaged in astrophotography, because doing so would destroy your night vision.
BUT, please do report back your findings, because it will be a useful data point.
The 5D3 is the most light-sensitive/noise-free DSLR that exists in a conventional-size (non-integrated-grip) body at the present time, AFAIK.
So it would seem, it's the best such DSLR for astrophotography. (Possibly barring the 60DA, but even then I suspect the 5d3 wins on low-light performance.)