I also have this problem since Periodic zone needs 2 mirror faces. That is why zone filled in Periodic and zoned filled in shadows are same zone. By the way, how to know where the shadow zone is? To not being the same faces zone, do name selection in meshing step should be done each face separately in order to get separate boundary condition ( I combined 2-mirror faces to be one boundary condition in order to define this periodic), but it doesn't work. Please suggest.
If this is MRF, does periodic bc's need to apply both inner and outer domain at the side?