I think result may be incorrect when putting small lumped element on large waveguide port. There must be transition from SIW to GCPW or microstrip line. Although it may be ok in HFSS. You can look into some HFSS waveguide examples. SIW port must be defined the same way as for rectangular waveguide. If you have difficulties making such 3d model and port, you can add small dielectric filled waveguide after last via of SIW, and then attach standard waveguide port to it. Or you are doing some kind of 2D schematic from HFSS microwave blocks?
You can check your result following way: make frequency sweep or change SIW length by one quarterwave. If your ports are ok, then impedance (or point on Smith chart) will be at the same position, with some insignificant change. If impedance value / point on a Smith chart changes significantly, then your ports have wrong impedance or defined incorrectly.