I'm building an EMV relay to educate myself on the protocol, but have hit a roadblock with the GET PROCESSING OPTIONS (GPO) command. Both SELECT(PPSE) and SELECT(AID) pass through fine, but my connection to the card times-out when I transceive a GPO command.

This happens both with a PN532 chip on an Arduino and Terminal Emulation on Android using the inbuild NFC chip.

I thought that the card might have some anti-relay system that is detecting the latency added by my relay, however a Proxmark sniffing a real transaction also failed to record any command-response pairs after the GPO command was issued, however the payment went through so (presumably) more packets were sent.

What could happen to my Visa Debit card after the issuing of the GPO command that means the card both entirely fails to respond to ISO-14443 compliant chips, that also hides any further communication from a proxmark, while still enabling communication to a real payment terminal?