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Cora relies on Apple Health and a local database to store its data. The data in these databases are automatically copied to your new iPhone when you setup your iPhone from an iCloud backup.
However, if you configure your new phone as a new device, these databases will not be copied to your new phone. This means that data you saved on your old phone are not accessible for Cora on your new phone. The only way to recover your data in this case is to manually log your past readings or to reinstall your new phone from an iCloud backup.