This is a change in behavior and the SYNC_LOGIN is now needed in order to utilize the 'Synchronize' button.
Link to the documentation
LDAP Service Availability for Login
In previous versions, the last valid LDAP password was saved in the AE DB to enable a login even if the service was not available at the time. The user passwords saved were also used for the synchronization available in the User object. This behavior is seen as a security issue and is therefore no longer supported.
Now, the LDAP service must be available for the login. Also, to use the Synchronize button in the User object for manual synchronization, a Login object must be assigned. If the LDAP service is not available, the access is denied. For more information, see UC_LDAP_EXAMPLE - LDAP Connection Variable.