Steps to Reproduce:

• Set User Email Visibility to be hidden
  • JIRA Administration > System > General Configuration > User Email Visibility
• Next, open an issue ticket and try to assign it to an user
• Notice that the list of Suggestions will show the string "undefined" (see attached screenshot)

Other Notes

• You can still properly assign the issue to the selected users.
• This only occurs for the Assignee field. It does not affect the Reporter field or custom fields.