They are using integrated the In-App Purchase module and they are testing it under Android within a game.
IAP module version: 1.11.4.

Problem: When they are using a Google Play promo code to buy a product, the purchase fails with "PurchaseFailureReason: UserCancelled".

Workaround: If we restart the app, ProcessPurchase is finally called and the product finally seems purchased correctly.

This looks very similar to what's described at https://forum.unity3d.com/threads/unity-iap-service-item-already-purchased-error.416175/

A similar bug was supposed to be fixed in version 1.8.0 (2016-08-23):
"Google Play - Promo code redemptions not being detected whilst the App is running."