wave scattering
Yes, you made a mistake within the design:
The Master/Slave Boundaries repeat the dielectric and the airbox, but not the PML.
Your model is an infinite dielectric plate with an infinite airbox (both in two dimensions) and right in the "middle" there is a small PML boundary on top and bottom.
Do the following:
Visualize the PML-boxes. Now you see a stack of three boxes: PML1, Airbox, PML2.
Create a rectangle on each side of the stack, covering all boxes. Then assign M/Sbound to these rectangles (and of course delete the old M/S).
So the PMLboxes are also extended infinitely.