Apple cider is acidic, so when you ferment all of that sugar, you're left with a fairly tart beverage. Of course the pH variable with what sort of varieties are in the cider. While brown sugar will leave some residual flavor, sugar is only going to dry it out more and enhance the acidity. Unless you kill off the yeast and back-sweeten the cider, that acidity is going to be quite present. Like most things, it should mellow a bit with age, but I don't think it's ever going to go away.