i can not help at all with HFSS.
but consider this, you could make an "antenna" out of an sma connector with a 50 ohm resistor chip soldered on.
the impedance match will be EXCELLENT.
but it will not radiate anything.
Antennas can have the same effect. Lets say you have a lossy material making up the antenna, and you need some sort of big standing wave/resonance to make it work.....it might appear to be matched, but also is radiating since it is just turning your RF power into heat