Masking Problem

[Doc_Pit]

Having a masking issue which I hope someone can explain to me.  The image below is from a snake painting.   It shows a piece of the image and the layer.  I want to put a soft, feathered gray stripe along the body of the snake.  I first used a hard brush to paint in a broad gray stripe.  I set the blend mode to multiply.  I attached a black mask.  I then used a soft white brush over the black mask to reveal the stripe.  I was able to use the mask properties to adjust the feathering until I had the appropriately soft edge.  There are two issues that puzzle me: (1) Notice that there are certain segments that are sharp rather than feathered.  One possible explanation is that I feathered the mask to a point where the opacity of the mask was insufficient to conceal the sharp edge of the original brush stroke.  However, if that were the case, I would expect that condition would obtain uniformly, not in segments.  (2)  If, for example, I use a black brush at full opacity to clean-up the gray that spills over the snake body (image, upper left), the black brush has no effect.  It does not hide the gray.  Same in the lower area.  A black brush will hide gray areas except where the edge is sharp.  There is no layer projecting anything from higher in the layer stack.  As expected, if I hide the layer, the gray disappears.  If I disable the black mask, the hard gray stroke is completely revealed.  I.E., everything works as expected, except for the issues I described.  The problem is easy to work around.  I'm just curious about what is happening here.  Thanks.

[c.pfaffenbichler]

Could you please post complete screenshots taken at View > 100% with the pertinent Panels (Toolbar, Layers, Options Bar, …) visible instead of snippets? 

Both of the whole image and the Layer Mask in question. 

[Doc_Pit]

If I posted a complete shot, I would be including nothing that further clarifies the problem, and it would be impossible to read my notations, which point out where the problem lies (sharp edge vs feathered edge).  

The salient points are that the mask is black, the brush is white, the underlying stratum is gray, the blend mode is multiply, the brush is at 100% opacity / 100% flow / 0% smoothing, no fade.  A hard brush was used to paint the gray and a soft brush was used to paint white over the mask.  I sincerely appreciate your response.

[Doc_Pit]

An added note:  Instead of painting a gray stripe with a hard brush, I added a gray clipping mask, so the entire body of the snake was gray.  I then copied the same black mask with the same white brush stroke to that new layer.  The issue with the sharp-edged/feathered edged segments disappeared.  It may be that my guess was correct:  In feathering the mask,  I reduced the mask opacity at the edges so that the hard-edged gray stripe showed through at some points.  Still doesn't explain why painting over those areas in black had no effect.

[c.pfaffenbichler]

If I posted a complete shot, I would be including nothing that further clarifies the problem, and it would be impossible to read my notations, which point out where the problem lies (sharp edge vs feathered edge).  

The salient points are that the mask is black, the brush is white, the underlying stratum is gray, the blend mode is multiply, the brush is at 100% opacity / 100% flow / 0% smoothing, no fade.  A hard brush was used to paint the gray and a soft brush was used to paint white over the mask.  I sincerely appreciate your response.

By @Doc_Pit

I asked for meaningful screenshots and you have not provided them. 

The snippet of the Layers Panels does not actually clarify the position of the affected Layer in the Layers stack for example. 

So please provide the file itself (feel free to remove all Layers except the affected one).