Shape builder leaves a faint stroke on solid (non-stroke) objects when subtracting
I'm having an issue with the Shape Builder tool where – no matter what I do, subtracting leaves a faint "stroke" from the object that lies "behind" the merged object. To illustrate this I added a cyan rectangle behind the object which was created with the Shape Builder tool by overlapping a cyan rectangle over the red rectangle and then subtracting the cyan segment.

These objects had no stroke and were pure solids. Despite this, I tried expanding the objects and expanding appearance beforehand but nothing seems to work and the faint non-stroke appears anyway. As it is, I can't really use the shape builder tool for anything.
Any assistance is greatly appreciated!
