User autocomplete not finding all users

Problem:
Some usernames aren't autocompleted when mentioning or assigning a task to a user.

Cause:
When adding Confluence to a JIRA Cloud instance, users are not properly migrated over.
This also causes Duplicates in the People Directory (covered by this bug CONF-9857)

Steps to reproduce:

1. create a JIRA only instance
2. add users
3. add Confluence to the instance
4. grant access to the users
5. try to mention a user that was created previously (before they access Confluence for the first time) accessing just JIRA won't update the 'content' table.
   Created users won't be displayed in the People directory although they are Confluence users with access.
   Once the user logs in to Confluence, the user is mentionable.

Expected: users are mentionable because they have Confluence access
Actual: users are not mentionable

Temporary Workarounds:

• Manually run a Content Reindex
  (due to CONF-33400 - Confluence Administrators unable to Reindex Manually please contact support at http://support.atlassian.com to run the Content Reindex)
• Login the user to Confluence
• Visit their Profile page manually by changing the username on another Profile page.