That explains and is consistent with the originalAppVersion definition:
https://developer.apple.com/documentation/storekit/apptransaction/originalappversion
originalAppVersion represents the CFBundleVersion (build number) on iOS, or CFBundleShortVersionString (marketing version) on macOS, for the original purchase; so unless build number is properly maintained (e.g., incremented if maintained as a number, or set to marketing version if maintained as a string), it won't be unique to identify past purchases on iOS (and for me almost all build numbers were 1).
Topic:
App & System Services
SubTopic:
StoreKit
Tags: