As part of making users aware of discussions, we will constantly use @{username} to notify a user.
This works for all users. However, we have one user that this is not working for. The user is active and in the same groups as other users that this works for.
Users are added to Jira/Confluence through social media login from our G Suite. We are not maintaining a specific SSO integration, just utilizing the authentication mechanisms of G Suite.
I'm not sure why we cannot address this user, but all others work.