I got the below code generated this amazing tool.

const size_t capacity = JSON_ARRAY_SIZE(2) + JSON_OBJECT_SIZE(1) + JSON_OBJECT_SIZE(4) + 2*JSON_OBJECT_SIZE(7);
DynamicJsonDocument doc(capacity);

JsonObject config = doc.createNestedObject("config");
config["type"] = "power_plug";
config["chip_id"] = "12345";
config["mac_address"] = "abc";

JsonArray config_attachments = config.createNestedArray("attachments");

JsonObject config_attachments_0 = config_attachments.createNestedObject();
config_attachments_0["name"] = "socket";
config_attachments_0["input_type"] = "toggle_switch";
config_attachments_0["on_state"] = "on";
config_attachments_0["off_state"] = "off";
config_attachments_0["pin_type"] = "output";
config_attachments_0["pin"] = 15;
config_attachments_0["current_state"] = "on";

JsonObject config_attachments_1 = config_attachments.createNestedObject();
config_attachments_1["name"] = "socket_1";
config_attachments_1["input_type"] = "toggle_switch";
config_attachments_1["on_state"] = "on";
config_attachments_1["off_state"] = "off";
config_attachments_1["pin_type"] = "output";
config_attachments_1["pin"] = 12;
config_attachments_1["current_state"] = "off";

serializeJson(doc, Serial);

This prints out the desired json properly. but when amend single line from the second array item, to take the on_state from my struct object d_config,

Serial.println(">>" + d_configs.peripherals[1].current_state);
Serial.println(">>" + d_configs.peripherals[1].on_state);
config_attachments_1["on_state"] = d_configs.peripherals[1].on_state;

the parsing gets quiet unexpected, and some of the keys from serialised string json gets missing. For above case, This gets printed in serial monitor;

>>off
>>on
{"config":{"type":"power_plug","chip_id":"12345","mac_address":"abc","attachments":[{"name":"socket","input_type":"toggle_switch","on_state":"on","off_state":"off","pin_type":"output","pin":15,"current_state":"on"},{"name":"socket_1","input_type":"toggle_switch","on_state":"on","off_state":"off","pin_type":"output","pin":12}]}}

NOTE in the second item of attachments, the key current_state is entirely missing. When I assign all values from this struct object, the serialised json misses many more keys... and the behaviour is quite unexpected. here is how I have defined my structs;

typedef struct Peripheral{
    String name;
    String input_type;
    String on_state;
    String off_state;
    String pin_type;
    String current_state;
    int pin;
};

typedef struct {
  String type;
  String chip_id;
  String mac_address;
  Peripheral peripherals[10];    
} DeviceConfig;

I am not sure if its an issue during serialization or something else... :/ Any help would be highly appreciated :) .