First off I am totally new to Inkscape and could be doing something wrong, Also I am sure I could combine this into one shape and eliminate the problem, But its strange
You haven’t done a boolean operation to knock out or join the shapes and the underlying fill . If you make three filled shapes and just lay them over each other, this will happen. One of the matrix tutorials goes through this issue.
Thanks @marmak3261 and @Jules. it is a neat effect, I think you could do some cool stuff with it. its also interesting that where there are 3 layers it did engrave, Just wanted to report it if it is a bug.
Yes it will alternate because a standard test to decide if a point is inside or outside a shape is to count the number of boundaries you cross going off to infinity. It doesn’t work for overlapping shapes.