Golang unmarshall nested dynamic JSON object in struct

Question

I have a JSON in the following format:

{
  "fieldA": {
    "dynamicField": "string value",
    "moreDynamicField": ["or", "array", "of", "strings"],
    "allFieldsAreDynamic": "values in str or arr[str] only"
  },
  "fieldB": {
    "dynamicField": "sameAsThoseInFieldA",
  }
}

The outer fields are fixed, but each outer field's value is a dynamic object. Therefore, I attempted to unmarshall this data into the following Go struct:

type JsonData struct {
    fieldA map[string]interface{}
    fieldB map[string]interface{}
}
func main() {
    var data JsonData
    json.Unmarshal([]byte(`{"fieldA": {"x":"x", "y":["x","y"]}, "fieldB": {"a":"b", "c":"d"}}`), &data)
    fmt.Println(data)
}

However, the above code produces empty Go array with no error: {map[] map[]}. I also tried fieldA map[string]map[string]any but the result is the same.

I didn't spot useful information on the json package documentation about this nested unmarshalling. Plus, will there be a performance drop using pure map[string]interface{} for the entire JSON data? Some benchmarking blogs and reddit tales gives contradictory conclusions...