ASP.NET Webforms Update panel is triggered only once

Question

So, the Regular Expression Validator cannot be made invisible or enable is equal to false. The images below describes everything.

 <asp:Label runat="server"
                    CssClass="col-md-2 col-form-label" ForeColor="White" Font-Size="Medium">NIC or Passport number:</asp:Label>
                <div class="col-md-3">

                    <asp:UpdatePanel runat="server" ID="updatePanel2" UpdateMode="Conditional" RenderMode="Block">
                        <ContentTemplate>
                            <asp:TextBox runat="server" ViewStateMode="Enabled" AutoComplete="off" ID="txtnic" AutoPostBack="true" OnTextChanged="txtnic_TextChanged" CssClass="form-control" />
                            <asp:Label runat="server" Visible="false" ID="lblnic" BackColor="White" ForeColor="Red"></asp:Label>
                            <asp:Panel runat="server" Visible="false" ID="pnltick2">
                                <lord-icon
                                    src="https://cdn.lordicon.com/oqdmuxru.json"
                                    trigger="in"
                                    delay="0"
                                    state="in-check"
                                    style="width: 30px; height: 30px; color: lawngreen; background-color: greenyellow">
                                </lord-icon>
                            </asp:Panel>
                        </ContentTemplate>
                        <Triggers>
                            <asp:AsyncPostBackTrigger ControlID="txtnic" EventName="TextChanged" />
                        </Triggers>
                    </asp:UpdatePanel>

                  <asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" Display="Dynamic" ControlToValidate="txtnic" ErrorMessage="Invalid NIC or Passport number format. Please check and try again." ForeColor="Red" SetFocusOnError="true" ValidationExpression="^[A-Za-z][0-9]{13}$|^[0-9]{8,12}$">
                  </asp:RegularExpressionValidator>

---

The code behind on Text changed.

        // Creating Connection.
        SqlConnection con5 = new SqlConnection(_conString);
        // Creating Command.
        SqlCommand cmd5 = new SqlCommand();
        cmd5.CommandType = CommandType.Text;
        cmd5.Connection = con5;

creating a parameterized query for the Username

        cmd5.Parameters.AddWithValue("@national", passwordEncryption(txtnic.Text.Trim()));

search for username from tbl_organizer and tbl donor

        cmd5.CommandText = "select don_nic_pass from tbl_donor where don_nic_pass=@national";

Creating DataReader to read from database.

        SqlDataReader dr;
        con5.Open();
        dr = cmd5.ExecuteReader();

        string input = txtnic.Text;
        if (Regex.IsMatch(input, "^[A-Za-z][0-9]{13}$|^[0-9]{8,12}$"))
        {
            RegularExpressionValidator1.Visible = false;
            RegularExpressionValidator1.Text = "";
            lblnic.Text = "";

Checking if username already exists in the DB.

            if (dr.HasRows)
            {
                pnltick2.Visible = false;
                lblnic.Visible = true;
                txtnic.Focus();
                lblnic.Text = "Nic Already Exists!";
                RegularExpressionValidator1.Visible = false;
            }
            else
            {
                lblnic.Visible = false;
                pnltick2.Visible = true;
                lblnic.Text = "";
                RegularExpressionValidator1.Visible = false;

            }
        }
        else
        {
            RegularExpressionValidator1.Enabled = true;
            RegularExpressionValidator1.Text = "(Invalid Nic/Passport number!)";
        }
        }

The nic is 12345678 existing

The nic is 123456789 NEW

[This should not happen because it is not match with if (Regex.IsMatch(input, "^A-Za-z][0-9] {13}$|^[0-9]{8,12}$"))