If you look at the events tab of the device. The “data” field is the value that is actually passed into the function. This is decrypted from the FRMPayload field in the encrypted packet in the gateways LoRaWAN frames which is decoded from the phyPayload in the MQTT message.
You can read more here: Manually decode PHYPayload or decrypt FRMPayload - #13 by nikospps