Mystery solved. Because the only fractions that can be precisely defined in binary are fractions with powers of 2 in the denominator, it turns out that floor(1.2/ 0.4)=2 and not the expected three. The MatterControl slicer must not allow for the imprecision and it assumes there are only two 0.4 perimeters and since there is no single extrusion gap fill (ala Simplify 3D) that I have found, it prints the two perimeters and leaves the gap. I have found that if a change the wall to be 1.21, the slicer sees three perimeters except when there are fillets on the corners. Using 1.22, the slicer also sees three perimeters for the corners.
This is not an elegant solution. One that might work universally is to set the nozzle to 0.39999999 in the slicer but I have not tried that.

]]>