Hooks for post saving make a post-new.php to load latest post’s data

Not sure what must be causing this. Try replicating your entire filter hook with function in this format:

function filter_handler( $data ) {
   // do something with the post data
   return $data;
}

add_filter( 'wp_insert_post_data', 'filter_handler', '10', 1);

Let me know if something happens.