Do you know why for some user you are able to write @ follow by the user name so he will get an email even the action is not assign to that person, but others are not able to do it.
Why?
The user should have "Browse Users" global permission before they can start picking other users.
Also see https://confluence.atlassian.com/jirakb/mentions-are-not-working-in-jira-applications-289277743.html
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