Cannot mention specific user

Issue Summary

When trying to mention a user ( with @mention or mention ) or finding them in the listed users of a user picker field (eg. Assignee) the user does not show up as an option. The specific user can't be mentioned by anyone on pages or comments. 

Steps to Reproduce

1. Go to advanced issue search
2. Search a user by Assignee filed or use JQL

   assignee = testuser
   

Expected Results

• You can find the user in the Assignee field in Jira

Actual Results

• The user doesn't show up as an option and cannot be mentioned

Workaround

• First one:
  • Contact our support team through https://support.atlassian.com/ and include the following information:
    The user(s) being affected
    A link to this repor
• Second one: 
  • Go to the site's administration > Users > Select the user > change the User Role to Trusted, refresh the page and then, change it back to the previous role (Site administrator/Basic).
• Third one (strictly for Jira)
  • Temporarily switch to the old issue view