EDIT: IT LOOKS LIKE THIS WAS A SCREEN GRAB ERROR WHEN DOUG
made the image to upload.
Most likely a graphics card redraw error. Defect not present if full frame jpeg. He is replacing the bad screen grab.
Viewed at 100% there is quite a bit of speckling noticeable around the text despite the moderate contrast.
See around the letters CHIE and the number 12. This is most likely due to interpolation to replace hot pixels.
There is also something else going on. On the 8 minute exposure the large letter R has an artifact that is larger and looks like
more than a hot pixel replacement. It looks like a vertical line has been replaced. There is also a similar horizontal one
just to the right of the letters SSI on the Classical sign.
While the speckling is more natural looking the vertical and horizontal "line" artifacts are rather nasty.
They are both absent in the 1 minute exposure and present in the 8 minute exposure. If these increase
even more for exposures over 8 minutes it could become quite problematic.
It would be interesting to see the dark frames as jpegs.
However with a 60MP file these artifacts will only be visible on big prints.
It also seems from the other crops that this problem does not show on more natural textures where interpolated
pixels to replace hot pixels will be less evident where there is more texture in the subject.
However it could be a problem with the typical high contrasts in city landscapes.
I have seen hot pixels with Canon and Nikon (D800) I have not seen these line artifacts.
Could this "line" artifact be column calibration that looks fine on a shorter exposure, but rears it's head on longer exposures?