add_filter('acf/load_field/name=room', 'populateRooms');
function populateRooms($field)
{
    $field['choices'] = array();
    wp_reset_query();
    $query = new WP_Query(array(
        'post_type' => 'room',
        'orderby' => 'menu_order',
        'order' => 'ASC',
        'posts_per_page' => -1,
        'tax_query' => array(
            array(
                'taxonomy' => 'room_category',
                'field'=>'slug',
                'terms' => 'vacant',
            )
        )
    ));
    $field['choices'] = array();
    foreach ($query->posts as $room_id => $matched_room) {
        $choices[$matched_room->ID] = $matched_room->post_title;
    }
    if (is_array($choices)) {
        foreach ($choices as $key => $choice) {
            $field['choices'][$key] = $choice;
        }
    }
    wp_reset_query();
    return $field;
}