Updating an Email Template is creating a draft for existing emails without the desired changes
When changes are made to an Email Template in the Design Studio that is already in use and those changes are approved, it will create a Draft for any existing Email that was created from that Template. If the draft is not reflecting changes you would like to see made, you may have included the changes within a mktEditable or Editable class <div> tag.
The content within a mktEditable class element within an email template are ignored from any changes as they are considered to be "default content". Any content within these elements will not see any changes made to them based on changes to the template, and will only be seen when creating a new Email asset.
You can update an editable section to remove it's editable status, however this will remove any changes to the content done in any email.
You can see which elements can be classified as Editable here: https://docs.marketo.com/display/public/DOCS/Email+Template+Syntax