OK, as of the latest OS build (19546) and/or "System Hardware Update - 11/21/2019" which appeared three days ago, the brightness issue discussed here seems to be fixed. Fingers crossed, but....
Do you reckon so? I've got that build and the hardware update and just opened now from sleep. Touching the slider on the brightness bar shows 84% but screen is dim. Moving the slider to 80% and normal brightness resumed. At least this build doesn't green screen like I was getting with 19041.
Update: the latest build wouldn't even install so decided to reset back to basics (not roll-back) but that would only reset back to insider 19546 (really?). Now reinstalled from stock 1809 download from MS site and brightness behaves itself
- so think I'll leave insiders on SPX for now.