RFID antenna multiplexer
You said the tags are 6 cm apart and I see you are using 13.56 MHz reader. Your antennas are conductive loops and they must be quite near each other. This introduces the possibility of mutual inductance occurring in the setup.
Moreover, you are disabling the unused coils by terminating them with 50 Ω resistor to ground. That will allow current to flow in the coils, due to Faradays law of induction. I would instead try disabling them by cutting the current flow, so an open loop instead of a load. If the current can flow, then your disabled coils are affecting the coil in use, changing the resonance frequency.