Wrong HFSS Simulation result

Hi everyone
I've simulated a coupled line band-pass filted using HFSS 2015. S11, S22, and S21 are reasonable and like my expectation, but S12 is irrationally deformed in passband. See below photos:

I used fast sweep, I used different Solution frequencies (in some cases the problem was solved).

Have anyone faced this problem and help me?
Thanks in advance

You have mentioned that at certain frequencies problem is solved and you have tried different setups as well so I can suggest you only one thing which can help is try to simulate with more refined mesh. Give it a try as well

It looks like another mode may be excited in that frequency range. Check to ensure that the same mode is being excited at all frequencies.

Is it possible to have same S matrix except S12 if another mode is excited ?

Not very likely, granted, but best at least check.

I'd also re-do the simulation with a discrete sweep -- any other types I wouldn't put a whole lot of faith in.