Unity web request POST not working.. but PUT is work fine

1. I made REST API server (spring boot)
   
   

2. use Postman. request "POST" work fine ( response : json )
   enter image description here
   
   

3. use Unity engine. request "POST" not work ( response : Bad Request )

IEnumerator PostRequest(Uri uri, string json, Action<string> callback)
{
    using (UnityWebRequest webRequest = UnityWebRequest.Post(uri, json))
    {
        webRequest.SetRequestHeader("Content-Type", "application/json");

        yield return webRequest.SendWebRequest();
        if (webRequest.isNetworkError)
        {
            Debug.LogError("Error : " + webRequest.error);
        }
        else
        {
            callback(webRequest.downloadHandler.text);
        }
    }
}

4. use Unity engine. request make it 'PUT" and change method to "POST" work fine....( response : json )

IEnumerator PostRequest(Uri uri, string json, Action<string> callback)
{
    using (UnityWebRequest webRequest = UnityWebRequest.Put(uri, json))
    {
        webRequest.method = UnityWebRequest.kHttpVerbPOST;
        webRequest.SetRequestHeader("Content-Type", "application/json");
        yield return webRequest.SendWebRequest();
        if (webRequest.isNetworkError)
        {
            Debug.LogError("Error : " + webRequest.error);
        }
        else
        {
            callback(webRequest.downloadHandler.text);
        }
    }
}

5. Why doesn't work request POST??