My display brightness is being reduced on screen locking but does not recover on unlock

I had issues with the recent Mesa fiasco and so ended up re-installing TW KDE which is fully working except my screen brightness is being dropped from 65 to 4 on lock and does not then recover when I unlock. I have seen Night Colour (?) is an issue but that is turned off and also I turned off dimming screen on the lock settings but still it happens every time.

Any suggestions as to what I look for now gratefully received.