By default, Restrict Content Pro applies discount codes to all payments within a membership, not just the first payment. For example, if a customer applies a 30% discount to a $10 subscription, the customer’s initial signup payment would be discounted to $7 and all renewal payments after the first payment would also be $7. There is an option to change this behavior and make discount codes apply only to the first payment in a membership.
Navigate to Restrict > Discount Codes and edit the discount code you want to change. There will be an option called "One Time".
When enabled, this discount code will apply to the first payment only. Using the same example as before, if a customer applies a 30% discount to a $10 subscription, the initial payment would be discounted to $7 but all subsequent payments would be for the original $10 amount.