Thanks for your reply. I just upgraded to 4.40, having discovered I was still on 4.21 But I think I'm probably not explaining myself properly.
I process the set of 13 images, and when I'm done, the focus through the depth of field is fine. But I have halos around some of the parts. So I find the source image where this is correct and select "Copy Source". I select a brush size of 30 and hardness of 30, with edge sensitivity at 0. I make a series of brush strokes, say 3 for example, and then make a mistake on the 4th.
My intent is now to eliminate the 4th brush stroke, leaving the first 3 alone. Pressing Ctrl-Z causes a repaint, but no change. I try "Undo last retouching operation" (third arrow from left). Like Ctrl-Z, it repaints with no change. So I try "Undo last Brush" - and all 4 brush strokes disappear.
I'm probably doing something stupid, but I can't get it right. What am I doing wrong?