Chances are this is the same thing I see with my Sig 213. When you are checking voltages of the batteries the screen and board are active and its pulling juice, one battery is being pulled more than others because well it's just not enough power usage to spread around. Because of that one of the you see the voltage sag of one of the batteries. The RX is only really going care about out of sync if the difference between the batteries is too high when firing and still only if it gets below a certain point. That battery is also going to be the likeliest to be off from the rest when removing the battery and putting them on a charger.