Image Shortcode from ACF ID

It’s because you have forgot to use $atts['post_id'] please find below code.

function acf_featured_item_shortcode( $atts ) {
    // Attributes
    $atts = shortcode_atts(
        array(
            'post_id' => '',
        ),
        $atts
    );
    $post_id = $atts['post_id'];
    $image = get_field('featured_item', $post_id );
    $size="medium"; // (thumbnail, medium, large, full or custom size)
    if( $image ) {
        $output = wp_get_attachment_image( $image, $size, "", array( "class" => "mx-auto d-block" ));
    } 
    return $output;
}
add_shortcode( 'acf_featured_item', 'acf_featured_item_shortcode' );