Can't change user mention display values

It appears the way User Mentions are displayed is driven off of the "User Display Name Attribute" in the User Directory Configuration, which we have set to "displayName" in AD. Our displayName attribute is in the format of "Lastname, Firstname" which doesn't look very nice when doing user mentions. And at this point, we can't change the format of our AD attribute.

So I followed this Atlassian article which describes how you can change your User Directory config to make Confluence instead use "givenname + sn" for displaying user's names (by putting in a bogus attribute). I did that, resync'd the directory with AD, and restarted Confluence and all the user's Profile pages now show a nice "Firstname Lastname" for their Full Name, which is great. But...

When I go to do a user mention with "@", they still show up and auto-complete using the old "Lastname, Firstname" format. How do we get User Mention's to reflect an LDAP user's new Full Name?

Note: If I create a LOCAL Confluence user, and change his 'Full Name' (and clear the User Properties cache), it's friendly name in the User Mention reflects the change correctly. Just doesn't seem to work with LDAP users.

I asked this on Answers, but haven't gotten an answer and I think it could be a bug. https://answers.atlassian.com/questions/329713/change-user-mention-display-name