Why ONLY the writable domain controller (RWDC) appear as “Name Server” in the DNS?
Why Read-only domain controller appears as “Host(A)” in the DNS?
*Note: This is a normal behavior
Reference:
https://awinish.wordpress.com/2011/10/04/rodc-read-only-domain-controller/
https://technet.microsoft.com/en-us/library/cc754956(v=ws.10).aspx