On the HubSpot ticket view, if there is an email already in the timeline, you can click "Reply" or "Reply all". However, these replies do not appear to be returned in the Engagements API. I'm currently using the ticket ID to get all engagements. Am I missing something or is this just not possible at the time being?
To clarify, are you passing the ticket ID to the Get associated engagements endpoint, or searching the Get all engagements endpoint's results for emails associated with the ticket ID?
If you share a link to the ticket you're working on as well as the request URL you're pinging, I'll take a look!