Dear @Ignacio.Lopez,
I would guess the eigenvalue you are getting is simply the result of numerical round-off in the calculations and azimuth averaging. The generator azimuth state should no stiffness, and in fact, should represent a rigid-body mode, with zero natural frequency.
Normally, we recommend eliminating the generator-azimuth state (keeping the generator-speed state) to avoid including the rigid-body mode in the linearization solution. This topic has been discussed several times on this forum, e.g., see: FAST linearization V7.
Best regards,