Treeview Within UserControl and UpdatePanel Does Not Do Async Postback on Expand, Just Select

Question

So here's the situation. We have a TreeView control within an UpdatePanel that we use. Everything works in our current situation; expanding via the + triggers an Async postback or selecting the text/icon also triggers an Async postback.

In order to use this TreeView and its functionality elsewhere, I moved all the code into a UserControl. The problem is, after I did this the + or icon clicking does NOT trigger an Async postback, only clicking the text triggers an Async postback.

The markup looks like:

<asp:UpdatePanel id="Panel" ChildrenAsTriggers="true" runat="server">
    <ContentTemplate>
        <uc1:MyTree id="myTreeViewUc" runat="server" />
    </ContentTemplate>
</asp:UpdatePanel>

<!-- Somewhere else on the page -->
<asp:UpdatePanel id="Panel2" ChildrenAsTriggers="true" runat="server">
    <ContentTemplate>
        <!-- Stuff -->
    </ContentTemplate>
    <Triggers>
        <asp:AsyncPostBackTrigger ControlId="myTreeViewUc" EventName="MyEvent" />
    </Triggers>
</asp:UpdatePanel>

The MyEvent is raised like this (in the user control):

public event EventHandler MyEvent;

protected void Page_Load(object sender, EventArgs e)
{
    this.MyTreeView.SelectedNodeChanged += OnNodeChanged;
    this.MyTreeView.TreeNodePopulate += OnNodeChanged;
}

protected void OnNodeChanged() {
    MyEvent(this, null); // raise custom event on node changed
}

If I got any syntax wrong, ignore it, it's pseudo code but it gets across what I am actually doing (raising the event on NodeChanged and Populate).

The current code we have simply does an AsyncPostBackTrigger on the MyTreeView control itself for SelectedNodeChanged.

TreeView code looks like:

<asp:TreeView ID="MyTreeView"   
    ShowExpandCollapse="true"  
    PopulateNodesFromClient="false"
    ExpandDepth="0" 
    ShowLines="false"      
    runat="server">
</asp:TreeView>