We switched to Apple's StoreKit 2 framework, which uses different internal plumbing. Not only is it the recommended way moving forward from Apple, but it improves the user experience for purchase management.

The issue is that, indeed, it actually validates that the App Store account currently logged in purchased the app. I believe this is intended behavior, as it closes a piracy loophole where you could log into the Apple ID of a friend, download all their paid apps, and then log in to a different account while retaining access.

If other apps don't run into this problem, it's possible they haven't moved on to StoreKit 2. Or the app has their own login independent of app subscription; setting aside the time it would take me to build that, I don't think most people are willing to create an account to try a camera app.