occ user:sync does not work for one user only

Description

When we do occ user:sync "OCA\User_LDAP\User_Proxy" then this syncs all the users from the LDAP server.
Running occ user:sync "OCA\User_LDAP\User_Proxy" -u <uid> should only sync user with id <uid>.

Steps

1. Setup LDAP server and add to owncloud.
2. Add users to the LDAP server (for eg. user1 and user2)
3. run occ user:sync "OCA\User_LDAP\User_Proxy" to sync all the users
4. Modify an attribute on the LDAP server for user1 (eg Display Name).
5. run occ user:sync "OCA\User_LDAP\User_Proxy" -u user1 to sync just for user1

Expected Behavior

The displayName attribute of the user gets updated in owncloud.

Actual Behavior

We get following output

➜ sudo -u www-data php occ user:sync "OCA\User_LDAP\User_Proxy" -u user1

If unknown users are found, what do you want to do with their accounts? (removing the account will also remove its data)
  [0] disable
  [1] remove
  [2] ask later
 > 0
Syncing user1 ...
Disabling accounts:
user1, User Onessss, user1@example.org disabled

And the attribute of the user doesn't get changed.

Start by reproducing the issue with the documented LDAP setup and the occ user:sync "OCA\User_LDAP\User_Proxy" -u user1 command. Trace the user-sync handling for the -u option and compare it with the reported disabling output; done means the selected user's changed displayName is updated without syncing all users or disabling the account.