Visual Flow

Y'all! The "Flow Limits and Considerations" section of H&T is insanely long and summing it up here would be a fool's errand. 

The tl;dr is that Flow is very much a work in progress and, while it's increasingly powerful, there are TONS of caveats and exceptions. 

In winter 20', Salesforce updated visual flow and made text template components rich text as well as plain text. As such, if you are editing an existing text template from pre-winter20' you will need to ensure the proper type is chosen.

Specifically, when using an email core action the text template must be plain text or else it will render the rich text as HTML in the email. To change this, select Plain Text from the drop down in the upper right.