that's not the TF of the PMC servo, it's something else. look at the gain level: -100dB. that's not more than some crosscoupling. Never trust a flat response, always think if the measured form and values make sense at all !
Take a minute and think about the form and values of the TF you expect from a servo like this. Have a look into the schematic and draw the TF shape of the individual gain stages and add them to an overall TF or use LISO to simulate it. Then measure parts of the servo step by step in order to verify that the individual parts are working as expected.
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 PZT notch box. It is a low pass filter. It's TF looks fine (I'll update it.)
However when the TF of the servo and the notch is measured together.