Almost embarrassed to post this here as it's bound to be something obvious but I'm away and it's driving me mad trying to find it (and just in case it's not)
The problem is when I'm using my Sony A1.
Basically I noticed the issue first when I put a 6 stop ND on for a photo (manual mode), I increased the exposure looking at the rear screen and histogram but it got to a point where increasing the exposure time did not affect the exposure on the rear screen, EVF or Histogram. After some trying I found that point was at 4 seconds. Anything over that the histogram etc stays the same. So if the photo was starting to clip at 4 seconds, but I increased the exposure to 30 seconds the histogram/rear screen would still show it starting to clip yet the photo taken would be completely over exposed. This will happen no matter what the ISO/fstop etc always 4 seconds.
Just recorded this to show: