Freehand mask tool Add expands mask well beyond existing border

I'm trying to use the freehand mask tool in Add mode to "smooth" a mask edge. Instead I get a considerable expansion of the mask near where the freehand addition was done. Below are 2 screenshots, the first showing the tool as a triangle  , the second showing the resulting mask just after a double-click of the tool. The freehand mask tool used to work very well and exactly but no longer does.