User Profile

I noticed a explain that "This account will be only used for testing your in-app purchases while developing locally. Your existing App Store account will be used for TestFlight apps" in iOS12。And our app has been rejected many times becouse IAP failed。Then we tested found that if use sandbox account in develop app or use real itunes Store accout in testflight app,it nonot has any probrom,only use real sandbox account in Testflight can be failed。so I want to know "TestFlight app test IAP does use sandbox account in iOS12"。