Pending subscriptions can't be confirmed on iOS

During testing on iOS we noticed a couple of subscription purchases that seem to be stuck in the pending state, unable to be confirmed. When Unity IAP initializes, it attempts to finish the transactions for these subscriptions, but when we call storeController.ConfirmPendingPurchase(product), Unity IAP tells us

"UnityIAP: Transaction [transactionId] not pending, nothing to finish here"

I'm guessing this is due to the fact that subscriptions keep getting new transaction Ids on iOS, which is something that caused us some issues when restoring subscription purchases (the transaction ID for the restored subscription was not present in the receipt due to being new, so our system thought the purchase was not valid).