any material interface forms reflection, and two such interfaces will form a cavity. I am not sure exactly what do you want to measure, but p2/p1 will change with distance, because inside the cavity it has standing wave. You need to develop a math formula for the fields or power transmission inside the cavity at different location.
If you want to know the mode property of such waveguide, a better way is to use FDE in MODE. You can also use mode source with "custom" selection to calcualte mode properties.
Or, you use long waveguide, and use the PML to truncate the waveguide to avoid refleciton. But I do not know the meaning doing this, as for straight waveguide, the mode property is sufficient.