What kind of attribute is this? I have tried a custom attribute:

and a preset/regular attribute (WooCommerce > Attributes):

and both show up on the invoice:

Turning on/off “Visible on the product page” didn’t make any difference.
Here’s the snippet I used:
add_action( 'wpo_wcpdf_after_item_meta', 'wpo_wcpdf_show_product_attributes', 10, 3 );
function wpo_wcpdf_show_product_attributes ( $template_type, $item, $order ) {
global $wpo_wcpdf;
if(empty($item['product'])) return;
printf('<div class="product-attribute">Custom attribute: %s</div>', $wpo_wcpdf->get_product_attribute('Custom Attribute', $item['product']));
printf('<div class="product-attribute">Preset attribute: %s</div>', $wpo_wcpdf->get_product_attribute('color', $item['product']));
}
I also tested your version:
$wpo_wcpdf->product_attribute('color', $item['product']);
Which works just as well.
Do you spot any differences?