Unable to re-purchase an app after reporting and getting refunded by Apple
It appears that this is by design. This is likely to avoid potential gaming of the App Store where users purchase an app, ask Apple for a refund and continue using the app.

Apple disallowed to update or re-download refunded apps on both App Store for iOS and Mac App Store. While this may hurt genuine use cases like yours where an app update has potentially fixed the problem.

Both pros and cons of the approach has been discussed. A few articles are linked below:


Apple makes it impossible to get updates for refunded apps
Apple now makes it clear you’re not allowed to update or redownload refunded apps


In case like yours, the recommended approach to take would be to file a bug report with Apple. It would also make sense to bring this to the developers attention.