[Vuejs]-Vue router removing query params

You are not returning anything inside map callback, assuming routes have to be returned, it should be like:

const items = (
    await fetchEntriesByContentType({ contentType: "items" })
).map(entry => {
    const routes = {
        route: `/items/item/${entry.fields.slug}`,
        payload: entry,
    }
    return routes;
});