Magento2 Order email missing custom variable values for first time & when I send email from admin it shows values

Magento2 Order email missing custom variable values for first time & when I send email from admin it shows values

Basically this custom variable values are the order attributes.

I have used this email_order_set_template_vars_before event to set the order attribute value in email. Not sure why it comes blank first time

checkout_onepage_controller_success_action is the event from one of my extension to send mail

Please guide