I have created an example. Looks like if we do refresh store for combobox, forceselection doesn't work:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="ABCImaging.BPOL.Main.WebForm1" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title>Ext.Net Example</title>
</head>
<body>
<form id="Form1" runat="server">
<ext:ResourceManager runat="server" />
<ext:PropertyGrid runat="server" Width="200" AutoHeight="true">
<Source>
<ext:PropertyGridParameter Name="Prop1" Value="prop1">
<Editor>
<ext:ComboBox runat="server" ForceSelection="false">
<Items>
<ext:ListItem Text="Item 1" Value="1" />
<ext:ListItem Text="Item 2" Value="2" />
</Items>
<Listeners>
<Select Handler="#{cbJopaStore1}.reload();" />
</Listeners>
</ext:ComboBox>
</Editor>
</ext:PropertyGridParameter>
<ext:PropertyGridParameter Name="Prop2" Value="">
<Editor>
<ext:ComboBox ID="cbJopa" runat="server" ValueField="Key" DisplayField="Value" ItemSelector="div.cmb-item" ForceSelection="false">
<Store>
<ext:Store ID="cbJopaStore1" runat="server" AutoLoad="false" OnRefreshData="SectionRefresh" >
<Reader>
<ext:JsonReader IDProperty="Key">
<Fields>
<ext:RecordField Name="Key" Mapping="Key" Type="int" />
<ext:RecordField Name="Value" Mapping="Value" />
</Fields>
</ext:JsonReader>
</Reader>
</ext:Store>
</Store>
<Template ID="Template6" runat="server">
<Html>
<tpl for=".">
<div class="cmb-item"><b>{Key}</b><br />{Value}</div>
</tpl>
</Html>
</Template>
</ext:ComboBox>
</Editor>
</ext:PropertyGridParameter>
</Source>
</ext:PropertyGrid>
</form>
</body>
</html>
protected void SectionRefresh(object sender, StoreRefreshDataEventArgs e)
{
var source = new KeyValuePair<int, string>[2];
source[0] = new KeyValuePair<int, string>(1, "1");
source[1] = new KeyValuePair<int, string>(2, "2");
cbJopaStore1.DataSource = source;
cbJopaStore1.DataBind();
}