ADS different attenuation of Coaxial model having same S21

Hi, I have strange problem about coaxial transmission line.

I design two coaxial transmission line using different components in ADS.
One component has different reference at each signal side. The other one has a common ground as a reference.

I focused on the S21 because I want to use the model to reflect the loss and delay of measurement in the transient simulation.
So, I matched the S21 and 50ohm Z0 of two designs as shown in the blow figure.

However, the transient simulation shows different attenuation.
The first graph in the transient simulation is that of the coaxial model having different reference at each side. I think this model is wrong.

The below figure is that having common gnd as a ref.
This model shows expected attenuation.

Why two model having same S21 show different result in transient simulation?
I really hope to find the answer!
Please help me.

Thank you!

If you look in the ADS help for the COAX element, you will see this note:

"Starting with ADS2005A, the COAX component is not available in the library or palette, but can be accessed from the Component History field. It is recommended to use COAX_MDS instead of COAX. The COAX component may cause an unexpected change in the transient response for some designs."

Not sure if that is related to the effect that you see, but it seems that the COAX model does have issue with transient.

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