Deserialize Json string and save in Text file using C# and Vb.Net in Windows Application

My code does (de)serialization of json strings. When user re-run the program, the previous data is fetched from txt file. I have two json string "json" and "json2", when I close and open program and look at txt file, json serialize and deserialze successfully while json2 is not.

I am new to json and c#, where is my flaw?

            public Form1()
            {
                InitializeComponent();
                input += ".txt";
                //default path + new filename
                path_combined = Path.Combine(root, input);    
    
                if (!File.Exists(path_combined))
                {
                    //File.Create(path_combined);
    
                    using (var stream = File.Create(path_combined))
                    {
                        if (new FileInfo(path_combined).Length > 0)
                        {
                            //flag situation
                            string json2 = File.ReadAllText(path_combined);
                            FormatJson(json2);
                            c1 = JsonConvert.DeserializeObject<Class1>(json2);
    
    
                            //Method used or not
                            string json = File.ReadAllText(path_combined);
                            FormatJson(json);
                            c2 = JsonConvert.DeserializeObject<Class1>(json);
                        }
                    }    
                }    
            }
    
            private void Button1_Click(object sender, EventArgs e)
            {    
                c1.flag = true;
                c1.flag2 = true;
                c2.M1 = true;
                string json = JsonConvert.SerializeObject(c1, Formatting.Indented);
                File.WriteAllText(path_combined, json);
    
                string json2 = JsonConvert.SerializeObject(c2, Formatting.Indented);
                File.WriteAllText(path_combined, json2);    
            }

            class Class1
            {
                [JsonProperty(PropertyName = "flag")]
                public bool flag { get; set; }
    
    
                [JsonProperty(PropertyName = "flag2")]
                public bool flag2 { get; set; }
                public Class1()
                {
           
                }
           }