Unfortunately, masks are not additive in the way you expected, when you have feathered them.

If you have only black or white pixels in your mask, you can duplicate and invert one, and know that every pixel in one mask or the other will be white, and show the corresponding pixel from its layer 100%. So every pixel in your resulting image will have a 100% contribution from a layer, guaranteed.

However, if you feather your mask, you end up with shades of gray. Let's say at the midpoint of the feathering, the RGB value of a pixel in the mask is mid-gray (128). If you invert that mask, that same pixel will also be 128.

But what happens when you apply two masks which are 50% gray? You unfortunately don't end up with the equivalent of a white mask, you end up with something more like 25% grey.

• Start with a white background layer.
• Add a blank layer, select a box shape and fill with black.
• Add a mask, and fill the mask with 50% grey. The "black" box will look 50% grey
• Duplicate the layer. The black box will get darker, but still not black
• Duplicate again. Still not quite black.

Here is a succession of 5 black squares, each with a 50% gray mask, with each layer shifted. As you can see it each layer progressively gets closer to black.

This is a simple demonstration of what you're doing when you invert feathered selections.

So inverting isn't going to work unless you have only fully black or white pixels in your masks. You'll either have to have sharp edges, or use some sort of dissolve technique that you can safely invert.

What I would do instead?

For each area I want to work on, just select a rough selection around that area and duplicate it to a new layer (saves space from duplicating the entire layer). Make your adjustments and create a mask to tidy up the selection. This mask can be feathered as you won't invert it.

Next area, do the same. Select (from the background layer), duplicate, adjust, mask.