To reproduce: use the following code snippet and follow the steps from the attached gif file. You will notice that the TextChanged event is not always fired.

        public RadForm1()
        {
            InitializeComponent();
            
            radDropDownList1.AutoCompleteDataSource = new string[] { "first", "second", "third", "fourth" };
            radDropDownList1.AutoCompleteMode = AutoCompleteMode.Suggest;

            radDropDownList1.TextChanged += radDropDownList1_TextChanged;
        }

        private void radDropDownList1_TextChanged(object sender, EventArgs e)
        {
            Console.WriteLine(this.radDropDownList1.Text);
        }

Workaround: handle the DropDownListElement.TextBox.TextBoxItem.TextBoxControl.TextChanged event