We have the same issue, sandbox purchases working fine, appstore purchases failing.
We think it’s because we’re using local validation and an older version of Unity IAP. Any app uploaded after the 14th of August will use new receipt encrypting and older versions of UnityIAP (older than 4.9.4) won’t be able to handle them.
See this thread: Important update: Support for in-app receipt validation for SHA-256 Apple Receipts.
UnityIAP 4.9.4 fixes local validation (search your codebase for usages of AppleValidator or CrossPlatformValidator).