Code to reproduce:

        public Form1()
        {
            InitializeComponent();
            _line1 = new Line(tChart1.Chart);
            InitializeChart(_line1);
        }

        Line _line1;

        private void button1_Click(object sender, EventArgs e)
        {
            _line1.Smoothed = !_line1.Smoothed;
        }

        class Student
        {
            public string Name { get; set; }
            public double Value { get; set; }
            public DateTime BirthDate { get; set; }

            public Student(string name, double value, DateTime birth)
            {
                Name = name;
                Value = value;
                BirthDate = birth;
            }
        }

        private void InitializeChart(Line age)
        {
            age.XValues.DateTime = true;
            age.XValues.DataMember = "BirthDate";
            age.YValues.DataMember = "Value";
            age.LabelMember = "Name";
            age.Title = "Children";

            void UseClass()
            {
                List<Student> list = new()
                {
                    new Student("A", 10, DateTime.Parse("1987-06-21")),
                    new Student("B", 5, DateTime.Parse("1992-12-03")),
                    new Student("C", 11, DateTime.Parse("1965-08-15")),
                    new Student("D", 14, DateTime.Parse("1979-02-09"))
                };

                age.Add(list as IList);
            }

            UseClass();
        }
    }

Clicking twice on the button causes data to be lost.