Global JsonObject (ArduinoJson)

I know a global JsonObject shouldn't be used, but I have my reasons.

I have a JsonObject* functions and I want him to save the content of a file. With

functions=new JsonObject(jsonBuffer.as<JsonObject>());

I can access the JsonObject, but not when I leave the method. Can someone help me to understand why? (Don't give me a solution like: copy into a struct, I need a JsonObject ready to be read in all my code.

Full code:

JsonObject* functions;

void loadFunctions() {
  File file = SPIFFS.open("/functions.json", "r");
  if (!file) {
    Serial.println("Failed to open the file");
  }else{
    size_t size = file.size();
    if (size > 512) {
      Serial.println("File size is too large");
    }else{
      std::unique_ptr<char[]> buf(new char[size]);
      file.readBytes(buf.get(), size);
      DynamicJsonDocument jsonBuffer(256);
      DeserializationError error = deserializeJson(jsonBuffer, buf.get());
      if (error) {
        Serial.println("Failed to parse file ");
      } else {
        functions=new JsonObject(jsonBuffer.as<JsonObject>());
        Serial.println((*functions)["test"].as<String>()); //this doesn't work in another method
      }
    }
  }
}