HFSS phase reference

Hello,

I have a coaxial line of length L. run it at frequency f. No dielectric just one PEC cylinder in a vacuum cylinder.
Defined waveguide ports.
since this is a TEM mode, I would expect the s21 phase to be 2*pi*L/wl.
It's not!

What am I missing?

Thanks

How is this a coaxial line? Have you confirmed the excited mode is correct?