Like I said: nothing should change overnight or on a restart. A large number of masks should not change that either. Lightroom shows you a preview and that preview should not be updated just because it restarted. If changes occur on restart, then there is something wrong. I offered two possible causes.
After some further fault finding I have discovered that the problem is caused by the automatic brightness setting on my Studio Display (see attached settings screenshot).
If set ON, the screen brightness starts at some default value (which is relatively bright, but not normally perceptible) then after about 30 seconds starts to slowly oscillate up and down for about 30 minutes before finally settling down to a value. This oscillation magnitude starts quite large, then slowly reduces (ie damped oscillation). For my edited photos in Lightroom this behaviour has a dramatic effect, though for the other windows and Lightroom panels etc on the screen it has no perceived impact. The photos initially display as overexposed, then slowly become underexposed, then overexposed etc until the display eventually determines its optimal brightness level. At this point the photos finally look like they were the day before. Whilst those oscillations are occuring there is no point trying to edit any photo as the results won't be reliable.
Since I have turned off the auto brightness the problem raised in this post has not recurred.
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