Specifically, after we had successfully purchased a subscription, an error was returned.

Please read the entire note!

Guideline 2.1 - Performance - App Completeness

We found that your in-app purchase products exhibited one or more bugs when reviewed on iPad and iPhone running iOS 13.5.1 on Wi-Fi.

Specifically, after we had successfully purchased a subscription, an error was returned.

Next Steps

When validating receipts on your server, your server needs to be able to handle a production-signed app getting its receipts from Apple’s test environment. The recommended approach is for your production server to always validate receipts against the production App Store first. If validation fails with the error code "Sandbox receipt used in production," you should validate against the test environment instead.