3-port directional coupler simulation: Directivity

Hello Guys:

can anyone help me to solve this question: how to define a 3-port directional coupler's directivity (4th port ISO is internally terminated).

normally, directivity is defined in such a way： D=S31/S32 (Port 1: input, Port2: output, Port3: Coupling port), with ISO port internally terminated. i have been puzzled by this definition for a while, here i give one example. (you can also see it in the attachment).
Case 1: image that the internally terminated impedance at ISO port is 50ohm. D=S31/S32.
Case 2: if i replace the 50ohm with a standard port, then i have 4-port coupler. so, D=S31/S41 according to the directivity in 4-port coupler.

the question is, in these two cases, nothing has changed. but why i get a so different directivity in these two cases? which one is the correct one?

Thanks