Hi,
Thanks for reporting this issue.
I am not exactly sure what you mean though.
Is the custom field sensei_payment_complete being added to the ticket email itself or to the order email?
Do you have a screenshot of what is happening that you could provide in a link to dropbox or a image sharing service?
Also, have you contacted WooCommerce about this as well? It appears their coding is adding it so they might have to fix it in Sensei.
Let me know what you find out and we can go from here.
Thanks