I’m using OTAA mode to activate my devices (SAMR34 custom board). I’m not sure what happens when there is a fCnt rollover, LoRaWAN sepcification don’t mention this. I found a comment here that says node should rejoin after there is a counter rollover.
I tried to test this by disabling the frame-counter validation (maybe this disables rejoin?), then transmitting uplink message with fCnt 65535 value. When I look up the values after the rollover I can see that app server counter is 65536, AppSKey and NwSKey are the same. Only sign that the counter has overflowed is that in Frame header fCnt value is back to 0 (well thats not suprising since FCnt size is only 16 bits)