Redeemed promo codes not included in updateTransaction

Question

I have an application that integrates In-App Purchases. The in-app purchasing functionality in general works as expected for purchases that occur in app, and is also fully tested to work for purchases initiated in the app store (tested using "itms-services://?action=....")

I am enabling my IAPService as soon as the app is launched in the AppDelegate, and it is correctly calling updateTransaction as expected, but....

When I redeem a promo code it does not arrive in updateTransactions. It does contain other purchases that I have made, but no mention of anything redeemed through the promo code (despite getting a lovely message on the AppStore stating the redemption was a success). In fact, it never arrives from any other subsequent launch, or even if I subsequently call:

SKPaymentQueue.default().restoreCompletedTransactions()

Any idea where I am going wrong? I have submitted a bug report to Apple in case it is something on their side but I am not holding my breath!