Good question: the modes you see should be the ones you expect. If you don't see the correct modes, then something is wrong with the setup (such as the waveport is not the correct size), or the solution is not sufficiently converged. In the analysis setup options, you can set the port impedance error to a much lower value (e.g., 0.1%) from its default value (2%). This usually helps lock onto the correct solution. Otherwise, try adjusting the size of the port.