When you sideload a book, it is processed the first time by the device. Information about the book is added to the internal database which is used for the library. The metadata is part of the information that is stored.
Unfortunately, the device doesn't have a way to update this information except by deleting the book and sideloading it again. But, if you use calibre, my Kobo Utilities Plugin has a way to update the metadata for books that are in the calibre library and are on the device.
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