Unable to @mention Customer or Request Participant

Summary

When adding a comment to a JIRA Service Desk request, the @mention feature does not work for customers or request participants unless those users have permission to browse the project (basically being considered agents) or if they have made a comment on the issue.

Steps to Reproduce

1. Install Service Desk
2. Have a customer create a request
3. As an administrator, view the issue in the Issue Navigator
4. Type a comment and use the @mention feature to mention the customer

Desired Results

• A list of suggestions will appear which you can choose the customer or all the request participant, regardless if they have made a comment or not.

Actual Results

• A list of users appears but this list does not contain the customer or any request participants unless they have made a comment.
  • Agents and collaborators do appear in the list. Any customers and request participants who are agents or collaborators will appear.

Workaround

• Add them manually using markup and Service Desk will notify them as if you had used the normal @mention method:

  [~username]
  

• Have the participant make a comment, and he will be mention-able after that.