The below described behavior happens on Microsoft Windows Server 2012 R2 Datacenter, I haven't tested it on older Windows versions.

Consider you have a domain + childdomain in 2 separate ip ranges across 2 sites



SiteA:

SiteA-DC01.root.local (10.10.0.1/16)

SiteA-DC02.root.local (10.10.0.2/16)



SiteB:

SiteB-DC03.root.local (10.11.0.1/16)

SiteB-cDC01.child.root.local (10.11.1.1/16)

Running dcdiag /e /v /test:DNS /f:output.txt from SiteA-DC01.root.local or SiteA-DC02.root.local will give you the following error (parts from output snipped): 



            DC: SiteB-cDC01.child.root.local

            Domain: child.root.local                             

               TEST: Authentication (Auth)

                  Error: Authentication failed with specified credentials

                  [Error details: 53 (Type: Win32 - Description: The network path was not found.) - Add connection failed]

     

               TEST: Basic (Basc)

                  The OS

                  Microsoft Windows Server 2012 R2 Datacenter (Service Pack level: 0.0)

                  is supported.

                  Error: Open Service Control Manager failed



                  [Error details: 1722 (Type: Win32 - Description: The RPC server is unavailable.) - Could not open Service Control Manager]



                  No host records (A or AAAA) were found for this DC

            -- snipped log --

         Summary of DNS test results:

        



                                            Auth Basc Forw Del  Dyn  RReg Ext

            _________________________________________________________________

            Domain: root.local

               SiteA-DC01                   PASS PASS PASS PASS PASS PASS n/a  

               SiteA-DC02                   PASS PASS PASS PASS PASS PASS n/a  

               SiteB-DC03                   PASS PASS PASS PASS PASS PASS n/a 



            Domain: child.root.local

               SiteB-cDC01                  FAIL FAIL n/a  n/a  n/a  n/a  n/a 

while running the command from SiteB-DC03.root.local gives the expected:



            DC: SiteB-cDC01.child.root.local

            Domain: child.root.local

               TEST: Authentication (Auth)

                  Authentication test: Successfully completed



               TEST: Basic (Basc)

                  The OS

                  Microsoft Windows Server 2012 R2 Datacenter (Service Pack level: 0.0)

                  is supported.

                  NETLOGON service is running

                  kdc service is running

                  DNSCACHE service is running

                  DNS service is running

                  DC is a DNS server

                  Network adapters information:

                  Adapter [00000010] Microsoft Hyper-V Network Adapter:

                     MAC address is 00:15:5D:0A:33:0C

                     IP Address is static 

                     IP address: 10.11.1.1, fe80::61fb:37f9:4f81:2d98



                     DNS servers:

                        10.10.0.1 (SiteA-DC01.root.local.) [Valid]

                        10.10.0.2 (SiteA-DC02.root.local.) [Valid]

                        127.0.0.1 (SiteB-cDC01.child.root.local.) [Valid]



                  The A host record(s) for this DC was found

                  The SOA record for the Active Directory zone was found

                  The Active Directory zone on this DC/DNS server was found primary

                  Root zone on this DC/DNS server was not found

            -- snipped log --

         Summary of DNS test results:



                                            Auth Basc Forw Del  Dyn  RReg Ext

             _________________________________________________________________



            Domain: root.local

               SiteA-DC01                   PASS PASS PASS PASS PASS PASS n/a  

               SiteA-DC02                   PASS PASS PASS PASS PASS PASS n/a  

               SiteB-DC03                   PASS PASS PASS PASS PASS PASS n/a  



            Domain: child.root.local

               SiteB-cDC01                  PASS PASS PASS PASS PASS PASS n/a 





         Enterprise DNS infrastructure test results:
    

            For parent domain root.local and subordinate domain child:

               Forwarders or root hints are not misconfigured from parent domain to subordinate domain

               Forwarders are configured properly from subordinate to parent domain 

               Delegation is configured properly from parent to subordinate domain



         ......................... root.local passed test DNS

It seems like there happens a NetBIOS fallback to resolve the child-domains DC in SiteB, which works from DC03 (same broadcast domain) but not from DC01 and DC02. This doesn't affect replication or anything else as far as i noticed, so the domain seems to be ok despite the dns test error. To verify that it seems to be the NetBIOS resolve issue, you can create an A Record for SiteB-cDC01 in the root.local Zone pointing to 10.11.1.1/16. After flushing dns caches (ipconfig /flushdns + clear cache in the DNS servers), the dcdiag will run successfull from DC01 and DC02 as well.