There appeared to be no pattern on which approval emails either did or did not include the Word template; a user would receive the Word template on to emails, then not on the third, but again receive it on the fourth.
Doing some searching online found a post by Rob Klaproth of Armanino where he had the same issue and confirmed that there is a known issue logged with Microsoft whereby when the user submits a document for approval, and clicks submit on the comment box before the Word template has finished generating the email will not include the Word template.
The workaround for this issue is to make sure that users wait until the Word template has finished processing, as shown by the Template processing message at the bottom of the main Microsoft Dynamics GP window. If they wait until this processing has completed before hitting submit, the Word template will be included on the notification email.
This issue affects all of the edit list Word template attachments through all of the Workflow approval types.