Issue Summary

This is reproducible on Data Center: (yes)

Steps to Reproduce

1. Set up a JSM project. Add some users to the project
2. On any issue, mention user-A (either a service desk team, admin or developer) in the same project. 
3. The user is not added to request participants

Note, this feature works as expected in the customer portal, just not within the agent / team view. 

Expected Results

The user should be added to the request participants. Based off this page: https://confluence.atlassian.com/servicemanagementserver0420/adding-request-participants-1095771829.html

Add participants by mentioning them

You can also add participants by mentioning them in the description or comments. They will be immediately added as a participants and will receive a notifications about it (if the Participant added notification is enabled).

• Agents: You can mention anyone in the project.

• Customers: You can only mention customers from your organization. In some projects, where customers are allowed to share requests with “any customer or organization”, you will be able to mention anyone in the project.
   

This feature was supposed to be introduced in 4.18 https://confluence.atlassian.com/servicemanagement/jira-service-management-4-18-x-release-notes-1072467369.html

@Mentions are here! 

Mentions are a great way to collaborate with your teammates, but they were never available on customer portals. Now, you can mention users in comments and issue descriptions in the same way as you would mention them in Jira, for example by typing @captainjoe. Mentioned people will also be added as request participants. Learn more about request participants

 

Actual Results

No user is added to the request participants page.

See example screenshot, admin adds mentions sample user Alana Grant (who is a service desk team member) and Alana is not added as a participant.   

Workaround

Request participants can be added manually.