The coupling between the antennas(-80,-90,-100 dB...) looks too low to make a good phase measurement. Phase measurement is sensitive to power levels. You should provide high enough power to make a clear phase measurement.
It seems that you make a mistake in your measurement.
Check your antennas, their frequency band, polarization etc. Start with better coupled networks, you can make the same measurement using a coaxial cable over a frequency band.
Place an attenuator and see how phase measurement is affected if you want. Increase the test power and reevaluate the phase measurement.