Please download the latest update (4.12.4) that is available on the App Store to fix the issue.
You can verify if you're really running the latest version in GoodNotes under Options > Settings > About. If it still shows 4.12.3, please make sure to follow the steps below to refresh the page.
You might need to refresh the App Store page first by swiping down with your finger or quitting the App Store app. Alternatively, you can also restart your device to refresh. It may take a moment to refresh but then the update button should show correctly and not falsely state "Open".