I don't see how you've measured it. If both sources are active, you'll get a superposition of voltages and currents, the ratio of which may not be representative.

It's fine to match a negative-real impedance into an opposite load. The conventional positive-real case, should have conjugate (opposite imaginary part), which cancels reactance, maximizing real power. In case real is negative, it can be canceled with parallel or series load [positive] resistance, and the reactive part still needs to be canceled. This gives a constant voltage or current source (depending on your perspective) and full power, rather than 1/4 power that the impedance matching theorem says in the usual case.

Naturally, negative resistance probably means you have a bigger priority, like conditional stability, or outright oscillation. Neither one of which you have much idea about in AC small signal analysis. You have to use transient for that to be sure.

The rather large Cgd sounds suspicious to me. Are you sure those parameters are representative?

Tim