Magento 2 Add custom test into Order Confirmation Email

In our requirement we’ve simple & grouped product, we need to add a product custom attribute in order confirmation email.

I’ve followed https://community.magento.com/t5/Magento-2-x-Programming/Custom-Attribute-into-Order-Confirmation-Email/td-p/459442 and implemented the same it’s worked as expected for the while order a simple product.

But while order a simple product which associated to the grouped product, the custom product attribut not included into order confirmation email.

You guys have an idea about the solution? your support is most appriciated one