In this scenario the EncodeScripts and RemoveScripts filters are disabled, so that script tags can be inserted. Using a script tag with some HTML content after it and submit when still on HTML mode, the content after the script tag will be removed. 

For the time being you can workaround this problem by forcing the editor to switch to Design mode on submitting:

<telerik:RadEditor ID="RadEditor2" runat="server" OnClientSubmit="OnClientSubmit">
</telerik:RadEditor>

<script>
    function OnClientSubmit(sender, args) {
        sender.set_mode(Telerik.Web.UI.EditModes.Design);
    }
</script>