I noticed that today to. After setting a static DNS Server on adapter level, the address seems to be replicated to the Key "ProfileNameServer" in the list of the interfaces in "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces" in the registry.
When switching back to DHCP, these values are still present. IMHO this seems to be a bug. After deleting those values and rebooting, the correct DHCP-given DNS server was used.