Payment method requested even though one already exists.

This may be an app-specific issue, but I'm using the "ProCam" app by Tinkerworks Apps, and this app offers a free in-app upgrade to allow me to use a function (zero mode). The upgrade works just like a regular in-app purchase, except for the price is $0. However, after I verify the upgrade with FaceID, I get a notification that says payment information is required. I already have 2 valid payment methods, and if I add a new one when it requests it, nothing happens and the upgrade doesn't register (but the new method is added anyways).

Is this an issue with the app or with the App Store processing?

iOS 26.1 (23B85)

iPhone Air (MG1J4LL/A - A3260)

ProCam 14.9.3