go version
go1.8.3 linux/amd64

go env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/xing/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build248645349=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

type Test struct{
	A string
	B string
}

func main(){
	a := Test{"12","23"}
	b := reflect.New(reflect.TypeOf(a)).Elem().Interface()
	c,err := json.Marshal(a)
	if err != nil {
		log.Fatal(err)
	}
	json.Unmarshal(c,&b)
	fmt.Printf("%#v", b)
}

result : 
map[string]interface {}{"A":"12", "B":"23"}

So we can see json.Unmarshal will change the type of 'b' from Test to map[string]interface{}

Reason :
encoding/json/decode.go function indirect (line 439)

if v.Kind() == reflect.Interface && !v.IsNil() {
			e := v.Elem()
			if e.Kind() == reflect.Ptr && !e.IsNil() && (!decodingNull || e.Elem().Kind() == reflect.Ptr) {
				v = e
				continue
			}
                        //maybe you can check the e.Kind() here
		}

Thanks