I believe I found a bug in the way masks are applied to groups. I've attached an example afphoto file to show the behavior.
Steps to reproduce:
Create white background
Create new pixel layer, fill with 25% gray
Duplicate 25% gray layer
Create two groups with the 25% gray layers
Select first 25% gray layer
Select "New Mask Layer"
Use gradient tool to create layer from white to black, top to bottom
duplicate gradient mask from above
Insert as mask for second group
Toggle the two groups to compare (turn off respective other group)
Expected result: the two images produced look the same
Actual result: the image when using the group mask has an uneven gradient
Here's the layer mask, looking as expected:
And here's the group mask, looking uneven: