f_t the problem you are having is likely related to your mesh properties. You need to increase the mesh until there is a cell border at the border between the conductor and the dielectric. Look at the mesh view, and check if the mesh is fine enough to distinguish between the conductor and the dielectric at the edge of the discrete port.
In general, you can simulate return loss with a waveguide port. If there are more than 2 conductors in a region you can use multipin waveguide. That said, sometimes a discrete port is the right port for the job so you will need to decide that.