I have the same issue with an ASUS Prime X570 Pro. Only the fan plugged into Fan 1 syncs. Also only about a 1/3 of the LEDs light up on the rear fan (FAN1).
Edit: I've entered a ticket at ASUS because I'm not certain whose issue this is.
I have the same issue with an ASUS Prime X570 Pro. Only the fan plugged into Fan 1 syncs. Also only about a 1/3 of the LEDs light up on the rear fan (FAN1).