The weird TF result from PMC seems to be the result of the insufficient voltage input. When I increased the swept sine voltage from 2mV to 500 mV, the result of the TF becomes as expected. See fig 1.
Before the signal is fed back to PMC, there is a PMC notch box. It is a low pass filter. It's TF looks fine (fig.2.)
However, when the TF of the servo and the notch is measured together, they look shaky.
I just read Frank's comment. I'll check the schematic of the PMC servo again.