Whenever you make updates to an email message, you also need to update the plain-text version if you would like those changes to also apply to the plain-text version of the email. To do this:
  1. Email > Email Campaigns > "Manage" next to the email campaign
  2. Go to the green "Messages" tab and next to the message in question click "Edit"
  3. Go to step 5, "Plain-text content"
  4. Here you will see what is currently set for the plain-text message. Click on the green button that says "Convert to Plain Text" to update this content.