The tooltip is not showing when RadFormDecorator decorate asp:CheckBox and asp:RadioButton. For example:

<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" />
  
<asp:CheckBox runat="server" Text="Update" ToolTip="Update Last Name" />
<asp:RadioButton runat="server" Text="Update" ToolTip="Update Last Name" />

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Removing the title attribute from the label element fixes the issue: 

<telerik:RadScriptManager runat="server" ID="RadScriptManager1">
    <Scripts>
        <asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.Core.js" />
        <asp:ScriptReference Assembly="Telerik.Web.UI" Name="Telerik.Web.UI.Common.jQuery.js" />
    </Scripts>
</telerik:RadScriptManager>
 
<telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" />
 
<script type="text/javascript">
    function pageLoad() {
        $telerik.$("label[class*='rfdRadio'],label[class*='rfdCheckbox']").removeAttr("title");
    }
</script>
 
<asp:CheckBox runat="server" Text="Update" ToolTip="Update Last Name" />
<asp:RadioButton runat="server" Text="Update" ToolTip="Update Last Name" />