Group membership is syncing for the majority of groups for a user but one group is not syncing membership for that user - and there are no errors.
If a user's primary group is set to a specific group other than Domain Users, that user's membership of that group will not be synced. This is because Active Directory does not include the primary group in the user's memberOf attribute, so a standard LDAP query will not return it as a group membership.
Set the user's primary group back to Domain Users. This ensures the previously affected group is no longer treated as the primary group and its membership will be returned by LDAP queries and synced correctly.
