Thanks for the information. Since my original post, I did some "experiments" at different iso settings. Best results were obtained at iso 320, and then increasing the exposure by four stops in Lightroom. The biggest difference compared to using a higher iso was that there was detail in dark foreground areas, while at high iso the dark areas were blocked. This is consistent with articles I've read about ISO invariance.

I do a bit of astro photography and do a lot of research etc. on best settings to use. I have never heard of anyone using ISO bracketing! Typically, higher ISO settings such as 1600, 3200 or 6400 are used and multiple light and dark frames are stacked to reduce noise. If you are worried about noise in dark foreground areas then just use a seperate exposure for them then blend the exposures in post processing. By the way, I also have an A7R5.

I would try resetting the camera. I can’t really think of anything else you can do other than try a different card.