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HMC424ALP3E Attenuation Response

I am testing the attenuation response of the HMC424ALP3E and am getting unexpected results past 16dB attenuation. My input signal is at 6.5GHz and the below graph details the response I get as the attenuation is changed from 0dB to 31.5dB. It seems that after 16dB the attenuator does not respond correctly. I am measuring the output power on a spectrum analyzer so I know the power being measured is only the power of the 6.5GHz tone and not any other broad band spurs or harmonics.

I have also verified the control voltages of V1 through V6 at each attenuation setting and the control voltages are correct in regard to the HMC424ALP3E truth table. The measured control voltage levels at the attenuator control pins are -1.9V for a logic low and -4.5V for a logic high. 

The bottom figure is a schematic of our circuit. The UP_V1 through UP_V6 inputs are connected directly to 3.3V digital control lines and the zener diodes have zener voltages of 5.6V (PN: BZX84C5V6TS-7-F).