In TestFlight, FaceID can not be used on the payment confirmation screen of the in-App purchase (Auto-Renewal) application. Is this a TestFlight specification?
All TestFlight apps operate in the sandbox environment. When you use what you think of as your production iTunes User account with a TestFlight app, you are in fact using a sandbox version of the account. The validation of all users in the sandbox environment is handled by a different server than for production operations. The use of FaceID as well as TouchID as an authentication method for iTunes and App Store purchases is an iOS feature for validating the user in the production environment. This is not a StoreKit feature. When a TestFlight user is reviewing your app, they will need to manually enter their account password when prompted to do so.
rich kubota - firstname.lastname@example.org
developer technical support CoreOS/Hardware/MFI