This is something that threw me for a loop for longer than I care to admit. When you are updating a Word doc to use with content controls that are updated from a Nintex workflow, you cannot just save the document with the "Save" icon in the toolbar depicted in the screenshot below:

You must use the File - Save As method and be sure to navigate to the library where the document is stored and overwrite the existing Word doc there.

This may seem elementary to some, but I'd like to document it to save others some trouble.