I did the other color pairs as well:
Why do the different combinations transition through these particular shades of gray, does that give us a clue as to what’s going on?
Here is “Mask: Simple Shape” ellipse with some softness rather than a hard edge.
The pattern holds again, pixels alternate from too light to too dark in the horizontal direction. This is hard to see on the top edge, which mostly matches the gradient made with GIMP. But the right edge is a different story, with the alternating light and dark bands being clearly visible.