Voltronic, pressure transducers don't have proximity effect; their diaphragms are exposed to the sound field only on one side, while proximity effect results from the continuing expansion of wavefronts as the sound from a point source moves forward in air. The path length to the back of the diaphragm in a pressure gradient transducer is longer and therefore, the nearer the sound source is to the microphone, the greater the relative "flattening out" (synchrony in their time of arrival at a flat surface) of the wavefronts as compared to the sound arriving at the front of the diaphragm. With pressure-gradient transducers it's always the difference between front and back that matters.
So the indicated rolloff is either electronic, or is due to built-in venting in the capsule gone awry. Pressure transducers are nominally sealed like barometers, but they always have some amount of very slow venting designed in, so that they can accommodate changes in ambient air pressure, and not risk bursting during air shipment where they might be in an unpressurized cargo hold.
--best regards