I'm using osTicket 1.16.3 on IIS with PHP 8.0.17
An agent only receives an email as 'last respondent' on the second time they respond in a ticket.
Under Alerts and Notices, "Last Respondent" is checked for New Message Alert.
1) User creates a ticket.
2) An agent responds to the ticket without claiming it.
3) User receives the reply and responds.
4) The agent will not receive an email with the latest user response.
5) Agent responds to the ticket again (still not claimed).
6) User receives the second reply and responds.
5) Now the agent is seen as last respondent and receives an email.