This is correct behavior. In VaultWiki 3, categories were in the Category namespace, so they all had "Category" before the name. In VaultWiki 4, there are no namespaces, so a category will be imported with a Prefix that has the same text as the old namespace name.
In VaultWiki 4, it is possible to create categories that have any Prefix you want, or no Prefix at all, so the Prefix becomes necessary to identifying the category. This was discussed in some of the videos Claw Snuff posted last year.