Why would WordPress use ‘attachment’ in a custom post type’s query string?

Why would WordPress use ‘attachment’ in a custom post type’s query string?

Albeit I have a slightly contorted structure in place, I can’t seem to find where ‘attachment’ comes into play and how to change that behavior.

My custom post type ‘fb_entry’ has the custom taxonomy ‘issue’. Posts from ‘fb_entry’ are rewritten with the slug ‘journal/%issue%’ where %issue% is the term name. I also have a second custom post type called ‘fb_issue’ whose posts are rewritten with the slug ‘journal’. There is also a page with the slug ‘journal’.

Here’s how this is intended to work:

1. User visits /journal
2. User browses list of posts from fb_issue and makes selection
3. User is taken to /journal/fb_issue
4. Here, user browses list of posts from fb_entry with ‘issue’ terms that match the fb_issue slug
5. User makes selection and views chosen post from fb_entry
6. Ideally, user has URL /journal/issue-term-name/fb_entry-post-name

This all seems to work fine until the final step whereat the user reaches a 404 page. Debug Toolbar tells me that the request is:

• /journal/issue-term-name/fb_entry-post-name

But the query string is:

• attachment=fb_entry-post-name

However, none of my queries call for attachments.

EDIT

Here’s the post type registration. Standard, I think:

Standardly, I think:

/* Create the 'Entry' post type */
function create_fb_entry(){
    register_post_type( 'fb_entry',
        array(
            'labels' => array(
                'name' => 'Entries',
                'singular_name' => 'Entry',
                'add_new' => 'Add New Entry',
                'add_new_item' => 'Add New Entry',
                'edit_item' => 'Edit Entry',
                'new_item' => 'New Entry'
            ),
            'public' => true,
            'rewrite' => array(
                'slug' => 'journal/%issue%',
                'with_front'  => false,
            ),
            'supports' => array(
                'title',
                'editor',
                'excerpt',
                'page-attributes',
                'revisions',
                'thumbnail'
            )
        )
    );
}
add_action('init', 'create_fb_entry');

function create_fb_entry_issue() { /* Create the 'Issue' taxonomy */
    $labels = array(
        'name' => _x( 'Issue', 'issue' ),
        'singular_name' => _x( 'Issue', 'issue' ),
        'search_items' =>  __( 'Search Issues' ),
        'all_items' => __( 'All Issues' ),
        'parent_item' => __( 'Parent Issue' ),
        'parent_item_colon' => __( 'Parent Issue:' ),
        'edit_item' => __( 'Edit Issue' ),
        'update_item' => __( 'Update Issue' ),
        'add_new_item' => __( 'Add New Issue' ),
        'new_item_name' => __( 'New Issue' )
    );  

    register_taxonomy( 'issue',
        array( 'fb_entry' ),
            array(
            'hierarchical' => true,
            'labels' => $labels,
            'show_ui' => true,
            'query_var' => true,
            'rewrite' => array( 
                'slug' => 'issue',
                'with_front'  => false
            )
        )
    );
}
add_action( 'init', 'create_fb_entry_issue', 0 );

EDIT 2

For further clarity, here’s the code I’m using to rewrite %issue%:

function fb_issue_permalink($permalink, $post_id, $leavename) {//   Fix dynamic 'issue'/'fb_entry' slug in 'fb_entry' permalink
    if (strpos($permalink, '%issue%') === FALSE) return $permalink;

        // Get post
        $post = get_post( $post_id );
        if( ! $post ) return $permalink;

        // Get taxonomy terms
        $terms = wp_get_object_terms( $post->ID, 'issue' );
        if( ! is_wp_error( $terms ) && ! empty( $terms ) && is_object( $terms[0] ) ) $taxonomy_slug = $terms[0]->slug;

        else $taxonomy_slug = 'unpublished'; // Substitute 'journal issue' in slug if 'journal entry' is not categorized

        return str_replace('%issue%', $taxonomy_slug, $permalink);
}
add_filter('post_link', 'fb_issue_permalink', 10, 3);
add_filter('post_type_link','fb_issue_permalink', 10, 3);