Suddenly unable to remove paint with 0 weight in Add mode or 1 weight in Subtract, same issue in Mix. It paints in all configurations of blend, weight, radius and strength. Gone through all the other stuff I can think of like lock modes, mirrors, shading, overlays. Vertex groups aren't locked. Tried reparenting and restarting Blender. I must have clicked something I shouldn't somewhere, because it's been fine all day.
So I'd emptied my vertex groups after making a mess of painting and had Auto Normalize selected in Tool Options when I started over. This seems to have prevented me from removing paint. I'm guessing all groups have to contain some weight before Auto Normalize can be employed?