Something must be wrong in your simulation setup. It is unlikely to be due to the fact that it is a multiport simulation (since under the hood a multiport simulation is just a bunch of "single" FDTD simulations). I would suggest you check the voxels of your simulation (View Voxels) and ensure that they accurately represent what you are trying to simulate. Pay special attention to the sources and the connectivity between the sources and the various PEC elements.
