We have a set of users that dont have the ability to do @username in the comment field and have the list of users show up. Same thing in the manage watchers in the JIRA issue... for some users the username typing generates a list of users but for a group of users it doesnt work.
Seems the group of people who are not able to use the @mention don't have "Browse Users" permission under the Global Permissions.
Please check the URL below and grant "Browse Users" permission to the affected users.
Hope it helps!
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