The issue is on the AD side:
There is a maxPageSize limit of 1000 in the windows AD which limits the LDAP query records to a max of 1000.
In simple words whenever an LDAP search or query is made to the AD, no more than 1000 records are returned from the directory. This limitation is actually a security design so as to prevent the AD from DOS attacks for LDAP queries. In case less than 1000 users are defined in the AD, no additional work is required but for a large setup this limit needs to be changed at the AD using the “ntdsutil” utility.