Help needed to understand HFSS ports - my simple model is wrong

I'm new to HFSS and find defining ports to be very difficult. In the example below, which consists of just one item (cylinder of dielectric) and a wave port, something is very wrong.

In this example, the model consists only of a cylinder of vacuum burried in a perfect electrical conductor. I've highlighted the port in the diamgram. Since this should act as a waveguide beyond cutoff at low frequencies, I would expect it to have a very poor return loss. Then as it starts to become a waveguide, I would expect it to have a poor return loss due to the fact that its shorted at the end.

What I'm seeing is a good return loss below about 9 GHz, then changing to poor above 10 GHz. I can't think of any logical explanation of why this would be so. Hence I've concluded my model is flawed. But this is about as simple as its possible to make any model - it only consists of one part.

Could some kind sole please take a look at this in HFSS, or could anyone explain why my reasoning is wrong.

Actually, HFSS is operating correctly. Your assumptions are not correct. When the mode is below cutoff, ie evanescent, then all of the energy going into the model decays evanescently before it reaches the pec cap at the end. Therefore, there is no energy that can reflect and exit back through the port, so the Return Loss (energy reflected back into the port) is near 0 % (or a very small - dB value). However, once the propagating mode is allowed, the energy is entirely reflected from the pec end and therefore all the energy is reflected back into the port, ie Return Loss should be 0 dB as the reflection power is 100 % (no loss as pec is implied).

Also, as this is a symmetric waveguide, there will be degenerate modes that need to be accounted for. Please see attached modified project.

Have Fun!