MB_Relationships_API::each_connected Returns Empty Array

Hi!

I'm not sure what I'm doing wrong. I've been on this for a couple of days and can't seem to get the API to return anything. Here's what I have.

I registered a relationship between Books (CPT) and Authors (CPT)

add_action( 'mb_relationships_init', function() {
    // Add CPT Relationship between Authors + Books
    MB_Relationships_API::register( [
        'id'   => 'books_to_book_author',
        'from' => [
            'object_type' => 'post',
            'post_type' => 'book',
            'meta_box'    => [
                'title' => 'Author',
                'priority' => 'high'
            ],
        ],
        'to'   => [
            'object_type' => 'post',
            'post_type' => 'book-author',
            'meta_box'    => [
                'title' => 'Books by this author',
            ],
        ]
    ] );
} );

On the Book archive page, I want to display the Author(s) name under the book title. I was actually able to do it using a query (like I would do on a single page) but I haven't been able to get it to work with the API.

When I add this to the archive-book.php page:

MB_Relationships_API::each_connected( array(
   'id'   => 'books_to_book_author',
  'from' => $wp_query->posts,
) );

And dump the content of $post->connected, this is what it returns: ARRAY (SIZE=0) EMPTY when it for sure has content attached to it. (Like I said, it works if I use the MB_Relationships_API::get_connected, but not MB_Relationships_API::each_connected.)

Is there something I'm doing wrong?