Why does my S11 parameter change when adding a 2nd inactive port?

When looking at off-normal reflection from a periodic structure, my S11 results change when I add a second, unexcited periodic port (S2x). My first periodic port is sending a TM mode in Hy, with amplitude H0 = E0/377[1/S], and thus ewfd.S11 represents the reflected TM response. My results look as expected.

I then added a 2nd inactive port to get results of a single field component (E = Ex, with amplitude E0), expecting ewfd.S21 to consist of just the results along Ex. However, this changes the S11 results! Why?

My only guess is if port 2 is absorbing the Ex component, thus port 1 collects only the remainder? (in this case, the remainder would be mostly Ez components, and possibly a small amount of Ey from chirality)

Thanks for your time and consideration on the matter!

-Bryan