wow... you just lost me in all of your wording.
let me get this correct.

You have a domain (public) and (private) using the same domain.name.
Example: abc.com and abc.com both public and private.

You perform an nslookup inside of your (private) domain and all seems fine?
nslookup (then get response). This response should be from your DNS located
inside of your network.

Now when you perform a nslookup on the outside public network you get what
your now calling wrong info?

(Q). On your registrant info, what do you have defined as your SOA
server(s). What I am asking is do you have DNS server setup on the outside
(public) such as ns1.abc.com and ns2.abc.com. You need to have this. As
it will point back to what ever DNS server is to be managing your domain
name.