Bug: Settings not available for custom fields

Bewilderingly, the values of a settings page are not available using rwmb_meta() at the time rwmb_meta_boxes filter runs. This means that a custom field cannot be populated by a callback that gets values from the settings page using rwmb_meta().

For example:

add_filter( 'rwmb_meta_boxes', 'brkthru_post_author_metabox' );
function brkthru_post_author_metabox( $meta_boxes ) {
    $meta_boxes[] = [
        'title'   => __( 'Post Author', 'brkthru' ),
        'id'      => 'brkthru_author',
        'context' => 'side',
        'class'   => 'brkthru-author',
        'fields'  => [
            [
                'name'    => __( 'Custom Author', 'brkthru' ),
                'id'      => 'brkthru_post_author',
                'type'    => 'select',
                'options' => brkthru_authors_array(),
            ],
        ],
    ];
    return $meta_boxes;
}

function brkthru_authors_array() {
    $authors = rwmb_meta( 'brkthru_authors', [ 'object_type' => 'setting' ], 'brkthru_settings' );
    $authors = array_column( $authors, 'author_name' );
    return $authors;
}

$authors is false in brkthru_authors_array() when the code runs inside brkthru_post_author_metabox(), but it works just fine if used elsewhere on the page.

I have resorted to using get_option() as a workaround.

function brkthru_authors_array() {
    // rwmb_meta() does not work here
    $settings = get_option( 'brkthru_settings' );
    $field_id = 'brkthru_authors';
    if ( isset( $settings[ $field_id ] ) ) {
        $authors = $settings[ $field_id ];
        $authors = array_column( $authors, 'author_name' );
        return $authors;
    }
    return array();
}