一个动态填充的DropDownList不会保留回发值，而其他ASP.net c＃

Question

如果没有回发，我有两个DropDownList填充在Page_load上。触发RadioButton回发后，第一个不保留其选择的值。

<asp:Panel ID="Panel1" runat="server" >
    <asp:Table ID="tblNew" runat="server" BackColor="#CBE8F5" Width="100%">
        <asp:TableRow>
            <asp:TableCell>
                <asp:DropDownList ID="cmbProveedor" runat="server" DataTextField="NombreCompañia" DataValueField="ProveedorNumero" CssClass="checkbox-round" Width="250px"></asp:DropDownList>
            </asp:TableCell>
        </asp:TableRow>

        <asp:TableRow>
            <asp:TableCell>
                <asp:DropDownList ID="cmbBanco" runat="server" DataTextField="banco_nombre" DataValueField="banco_id" CssClass="checkbox-round" Width="250px"></asp:DropDownList>
            </asp:TableCell>

            <asp:TableCell Width="50%" HorizontalAlign="Left">
                <asp:RadioButton ID="chkNormal" runat="server" Text="Normal" GroupName="tipo" OnCheckedChanged="chkTraspasos_CheckedChanged" AutoPostBack="true"></asp:RadioButton>
                <asp:RadioButton ID="chkTraspasos" runat="server" Text="Traspaso a Banco" GroupName="tipo" OnCheckedChanged="chkTraspasos_CheckedChanged" AutoPostBack="true"></asp:RadioButton>
            </asp:TableCell>
        </asp:TableRow>

在回发时，第一个DropDownList不会保留其值，而第二个。

    protected void Page_Load(object sender, EventArgs e)
    {
        client_id = Request.Cookies["ClientId"].Value;
        corp_id = Request.Cookies["CorpId"].Value;
        company = Request.Cookies["EmpresaId"].Value;
        user_id = Request.Cookies["UserID"].Value;
        C = new Tesoreria.Cheques(company);

        if (!Page.IsPostBack){

            P = new Proveedores(company);
            P.Get_proveedores(client_id, corp_id, company, user_id);
            cmbProveedor.DataSource = P.GetData();
            cmbProveedor.DataBind();

            Banco = new Tesoreria(company);
            Banco.Get_Bancos(client_id, corp_id, company, user_id);
            cmbBanco.DataSource = Banco.GetData();
            cmbBanco.DataBind();