Why is S11 measurement changing on Agilent E5071B Network Analyzer?
You are measuring various values, because the effective value is changing as the power harvesting conditions change.
The RF arrives at a number of diodes. These diodes turn on and off differently depending on the bias voltage that they're feeding, and the current that's flowing through them. Very hand-wavingly, the impedance varies with current. Low current, high impedance. High current, low impedance.
The changing S11 is perhaps a sign of an efficient rectifier. It would be easy to make the S11 change less with power conditions by adding an attenuator pad at the input!