There is a policy, usually at the domain level, that sets this and it supercedes DNS IP issued by DHCP or manual entry.
To check for existence of this policy, look in the client's registry for the
NameServer entry of type SZ (string) with the ip address the client is using for DNS.
If it exists, the policy may need to be reveresed (Disabled, not "Not Configured") or corrected at the DC.
From the group policy:
Computer Configuration...Administrative Templates...Network...DNS Client...DNS Servers.
Set the policy to either "Disabled" or the correct IP address, but do not set it to "Not Configured" or no changes will take place.
Note: This policy may be set somewhere other than at the domain level. Use GPResult or rsop.msc to find the source location.