Why are my Email 2.0 template variables not displaying in the text-only version of my emails?

Issue Description
When I add variables to my email template assets, the value assigned to that variable is not showing in the text-only version of the email even though it shows in the HTML version.

Issue Resolution
Email 2.0 variables do not pass the defined values for variables through to the text-only email. Since Marketo token values are passed through to the text-only email, customers may expect variable values to also be passed through to text-only emails. However, this is not currently the case.