Mentions are broken when you changed the User Name Attribute" from sAMAccountName to userPrincipalName

Summary

When using userPrincipalName as the User Name Attribute for AD LDAP configuration in JIRA, mentions will be broken.

Environment

JIRA 7.3

Steps to Reproduce

1. Setup AD user directory in JIRA using the default configuration
2. Mention a user in the comment of a test issue
3. Change the User Name Attribute from sAMAccountName to userPrincipalName
4. Go back to the issue

Expected Results

Mention of the user is working fine and correctly

Actual Results

Mention of the user is broken

Workaround

Try not to use userPrincipalName for the User Name Attribute