I put the camera in manual exposure mode and set it to 1/1000 at f4. Sometimes after I take a shot, it changes the exposure to something like 1/800 at f3.5. Is there a setting that is doing this? I have another D800 that always stays where I put it. But it seems like this camera is set to "make a suggestion" for me which I don't want.
I have never seen this kind of behavior and I am not aware of any setting that would impact S/A set by user in M mode.
The only thing I can think of are the 2 separate banks of settings between photo and film in live view, since each of them keeps their own setting. Did you not change that mode by mistake?