Stabindex and S1/S2 in Sprobe Cadence

Hi,

I added Sprobe to the LNA I designed to check the stability of interstages. But the results generated by Sprobe confuse me.

For StabIndex, it is always less than 1. However, at some frequency, S1 and S2 will be greater than 1, and Z1 and Z2 will also have negative real impedance at that moment. I am wondering whether the circuit is in the unconditional stable state or unstable in this case?

Thanks!