Adafruit Fingerprint Sensor library: issue with loadModel

I'm using the code in https://forum.arduino.cc/index.php?topic=556857.0 to try to find the empty slot for enrolling a new fingerprint, but running in to an issue. Every time, it tells me ID 1 is available, even though there is currently a fingerprint there (have seen it match when I scan my finger).

I dug in a little more and loadModel returns FINGERPRINT_PACKETRECIEVERERR. Otherwise, the fingerprint reader works fine. Can enroll (keeps overwriting ID 1), can match for access. Any ideas?