P: Add/Subtract to mask using Brush does not allow L&R Brackets to adjust brush size.

If I make a gradient mask adjustment, then select subract using brush, the left and right bracket keys no longer manage the brush size but instead increase or decrease the star rating of the image.  The brush mask itself works fine with the bracket keys changing the brush size.  It seems only to happen when the brush is being used to modify an existing mask, although I did not test beyond the example cited.  Is there something I'm doing wrong or is this a true bug?