I have same issue BUT:
setting SF10 results in 100% joinAccept reception by end node
setting SF11 results in 50% joinAccept reception
setting SF12 results in 0% joinAccept reception and failure to join.
Tried using ABP with confirmed messages. Node sends packages with confirmation request. Again
SF10 - 100% confirmed
SF11 - 50% confirmed
SF12 - none is confirmed
Raising SF causes end node to fail receiving downlink packages. My end node is class A.
I suppose SF12 should be more stable…
What might be the reason?