Issue (bug?) with useSWRInfinite and initialData

[grazianodev]

I'm trying to use 'useSWRInfinite' with SSR in a Next.js project. The following component fetches a list of posts, with the first page of posts ('initialPosts') fetched on the server:

export const DynamicPostsList = props => {

 const { cacheKey, fetcher, initialPosts } = props

 const getKey = ( pageIndex, previousData ) => {
    
  if( pageIndex === 0 ) {
   return [ cacheKey, initialPosts?.pageInfo?.endCursor ]
  }
  
  return [ `${cacheKey}`, previousData?.pageInfo?.endCursor ]
  
 }

 const { 
  data, error, mutate, size, setSize, isValidating 
 } = useSWRInfinite( getKey, fetcher, {  
  initialSize: 0, 
  initialData: initialPosts ? [ initialPosts ] : [],
  revalidateOnFocus: false
 })

 return(
  <div>
   { data && <PostsListCore { ...props } posts={ data } /> }
   <button onClick={ () => setSize( size + 1 ) }>Load more</button>
  </div>
 )

}

My problem is that when I click on 'Load more' the first time, my fetcher correctly fetches the second page of posts, however the new posts are not appended to 'data', so I still only see the first page of posts (fetched on the server). Am I doing something wrong or could this be a bug? Shouldn't 'data' be updated to include the second page of posts too?

If I click on 'Load more' again, the app fetches the second and third pages of posts, which replace the first page. After that, clicking on 'Load more' works as expected, i.e. one page a time is fetched.