antenna return loss measurement question

Dear colleagues;
I am working on wifi and bluetooth boards. And try to measure antenna isolation between antennas.
I applied the proper coax cable to vector network analyzer (vna) and perform the calibration. Then connect the coax cables first to wlan antennas (2x2 system).
Result is seen as below;

Then remove one of the port cables and connect to Bluetooth antenna. Then result is seen in below figure (yellow bt antenna, purple wlan antenna);

Here my question, why wlan antenna return loss result is different from the first measurement? I was expecting to see a similiar purple graph as in the first figure.
I tried to use 2nd port as open, short and load (instead of connecting to bluetooth antenna), but result is same.
I can't comment on that and want to ask senior guys.

Any explanation?
Thanks

Antenna which not are under measurement must be properly terminated (50 Ohm).