I can see in the audits of my ticket that the trigger fired and sent a notification to the customer. However, when the end user checks the email, there is just a canned response:
Why are notifications sent to customers without the content of the ticket?
By itself, the trigger notification does not deliver the content of your tickets. Add a comment placeholder in the trigger actions that renders what you have to say.
For an extensive list of all the comment data placeholders, see the article: Zendesk Support placeholders reference.