Hi again @micropuller
the 2 images show the wiring for flashing the firmware, right?
I found this guide: how to flash Attiny using Arduino
The Qwiic RFID Reader has the chip ATtiny84A
Another user @paulvha figured out that there is a bug in the firmware that likely causes the bug you and I stumbled upon. He describes it here: Qwiic RFID wrong readings after 20 reads - #8 by paulvha