Edit Text Version of the Email without Affecting HTML
Issue
Sometimes, you may notice that editing the text version also affects the HTML version of the email. Is there a way to edit the text version without the same edits occurring to the HTML version?
Solution
Changes to TEXT never apply to the HTML version.
It's likely because when you "save" an element, it will add the "root block element" to the HTML side if one isn't present. So, your HTML is probably not handling it correctly. You can change the "root block element" to none in the Admin > Email > Editor Settings dialog.
https://docs.marketo.com/display/public/DOCS/Using+the+Rich+Text+Editor