Select Subject - Why can't I paint black on mask?

I usually extract subjects by slowly masking out the background but I decided to try speed up the process by using Select Subject and then using Select and Mask to make refinements.

I then like to save as a selection with a mask so I can make further refinements using the brush tool.

I have found in two images I worked on this week a horrible anomaly when trying to paint black with a 100% hard brush set at 100% Opacity and 100% Flow to try and sharpen the edges on the black mask. It's as if the brush is working at a lower opacity than 100% (I don't have pen pressure on).  You can see this in the image I've included.

I will probably revert back to the slow process of hand painting accurate, sharp masks with my Wacom pen but is there something I'm missing?

It's rotten and I've found it shows up on subsequent layers.

Thoughts appreciated.