In-App Purchase doesn't use double click side button to confirm payment

Hi.

I set up In-App Purchase for my app. I test on the hardware device (iPhone X) with a sandbox user.

All seems to work fine, except that the the app doesn't use the double click feature on the side button to confirm the payment. Instead it presents a "Purchase" button on the payment sheet, and asks the sandbox users' password again (credentials were already entered previously).

Is this normal?

I must be sure the double click side button feature is properly used in the final released/appoved App Store version.

Many thanks,