Actually I don't see perfect reflection (input or output) below 20GHz.
Anyway, as I already mentioned, your S11, S22, and S12 looks fine. Maybe S12 (isolation) is too good to be true at this microwaves frequencies, and following this information may give you a hint why S21 (gain) is negative.
The band-pass shape of S21 shows that you might use a high-pass input matching and a low-pass output matching networks.
Not to the last, check if your active component is still alive, even you get some collector/drain DC current.
Would be good if you can post your schematic.