Backed or not, there never was the promise that you could redeem for gold, nonetheless any specified weight in gold (e.g., N grams)

Otherwise people would buy the discounted currency up, and redeem it, and the price of the remaining currency would rise to about the value of that weight of gold the currency represents (less redemption fees, and other costs), unless the discount is because you cannot redeem on demand, at a counter, and carrying a redeemable note might not actually yield the weight of gold (if anything) as expected.