If you want to show order attributes in the email, there is need to insert variable to the email template content.
1. In the templates "New Order" and "New Order for Guest" please use the following variable:
{{block class="Magetrend\OrderAttribute\Block\Sales\Order\Email\Attributes" source=$order}}
2. In the templates "New Invoice" and "New Invoice for Guest" please use the following variable:
{{block class="Magetrend\OrderAttribute\Block\Sales\Order\Email\Attributes" source=$invoice}}
3. In the templates "New Shipment Order" and "New Shipment for Guest" please use the following variable:
{{block class="Magetrend\OrderAttribute\Block\Sales\Order\Email\Attributes" source=$shipment}}
2. In the templates "New Creditmemo" and "New Creditmemo for Guest" please use the following variable:
{{block class="Magetrend\OrderAttribute\Block\Sales\Order\Email\Attributes" source=$creditmemo}}