Per Model Settings -> modify settings for overlap -> infill only
Never works, and you always have to manually set everything (outer wall, top, bottom layers) to 0 before you can get the expected result as shown in second image:
Here in the second img, I set the density to 3%, and that checkbox that says Infill Only is completely inconsequential. The expected behavior for having infill-only should be as seen in the second image.
I think I know what it's doing in the first image too. Cura interprets this support blocker object as a separate stl and tries to generate its own up/bottom/outer skins, and inserts it inside the main mode. This should honestly be off if InfIll Only is turned on. An easy fix would be to turn all 3 skin options to 0 like I have shown in the second image IF the checkbox is on.