"Redirect URL on Success" field is not writing to database

The "Redirect URL on Success:" field under the "Payment Types" tab in the Directory plugin settings is not saving to the database when I enter in a custom URL for Paypal to redirect to after a successful transaction. It always redirects to "/listings/my-listings/" no matter what.

How can I fix this issue? I've already tried putting the direct URL inside "directory/core/paypal_express_gateway.php" as a value for "return_url" and "payment_url". But it continues to send users to the default page.

Please help!