resonance in a transmission line

I have met a problem recently and hope experts can give me some hints.

You can find the resonance frequencies from the S parameter curves of a transmission line (eg. microstrip line as shown in the following figures). When both sides of the transmission line are attached with a PMC(perfect magnetic wall) wall, how will the resonance frequencies change? Lower or higher?

From my original point of view, the transmission line without PMC has fringing capacitor effects, so its resonance frequencies should be lower than those with PMC. However, from the HFSS simulation results, the picture is different. The resonance frequencies of the transmission line without PMC are larger than those with PMC! (the following curves are from HFSS) I don't know why and which one is correct. Who can help me clarify this problem? Thank you very much in advance.

The crosssection of the transmission line.