Problems with IAP in old app

My company did an app for a customer 10 years ago. The app has in-app purchases (not subscriptions). It is an Objective-C code base. Over the years we've done minor updates to this app in order to keep it in compliance, but no major overhaul.

The app has been pretty reliable in the past, but recently my customer is getting a flood of complaints from users who say their in-app purchases disappear and can't be restored. It's happening too often to just be a case of bad internet connection or similar issue. The only thing that seems to be consistent is that it's never US-based users.

Whenever I test this app using sandbox accounts, I cannot reproduce the problems that are being reported. I'm always able to make purchases and restore them after reinstalling the app, or restore them to a different device. The content associated with the purchases always downloads correctly, even if I intentionally interrupt and restore internet access.

What can I do to figure this out? Should an IAP based app from 2014 be expected to work correctly today? Are there new legal issues with non-US app stores that we need to be aware of?

I realize there isn't a lot to go on here, but I don't have any more information.

Thanks, Frank