I sent in a case to Digi support but never got any response back.
Luckily for me recently I stumble across an option on the end node and found a solution. If you set JV=0, it will stop end node from looking for parent to join. You might have to do a network reset (NR=0) on the end node if you bring in a new coordinator of the same PanID. New coordinator might choose a different channel.
According to Digi, JV only apply to Router. But End node seems to somewhat behave like a Router if you set JV=1.