Asp:Button click event inside update panel not firing

What would cause an asp Button not to fire after a partial update has occured?

<asp:UpdatePanel ID="upPan" runat="server" UpdateMode="Conditional">
  <ContentTemplate>
    <asp:Button ID="btnSave" ClientIDMode="Static" runat="Server" Text="Save" CausesValidation="false" />
  </ContentTemplate>
</asp:UpdatePanel>

The first time any button is fired inside the update panel the below routines fire in this order.
After a postback has occured triggering the button again causes a postback, both Load and PreRenderComplete events fire but the click event is skipped.

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.PageLoad
  //Runs everytime
End Sub

Protected Sub btnSave_Click(sender As Object, e As EventArgs) Handles btnSave.Click
 //Doesn't fire after first postback.
End Sub

Protected Sub Page_PreRenderComplete(sender As Object, e As EventArgs) Handles Me.PreRenderComplete
  //Runs everytime
End Sub

FAILED RESOLUTIONS

Suggestions to resolve this include:

ChildrenAsTriggers= "True" This is already the default behaviour of UpdatePanel and offers no change.
<asp:AsyncPostBackTrigger ControlID="btnSave" EventName="Click" /> Again by default child controls of the panel cause an asynchronous postback and declaring triggers is redundant.

SUCCESSFUL RESOLUTION

If I simply change the asp:Button into an asp:LinkButton the issue is resolved.

SUMMARY

The postback is occuring but the click event is being missed when the sender is an asp:Button control.

Can anyone explain what would cause this behaviour?

Solution

First up, apologies my answer is in C#. It's also not much of an answer as I can't replicate your issue. The distinction between Buttons and LinkButtons is that a Button uses submit behaviour and a LinkButton uses a javascript postback. You could try putting UseSubmitBehavior="false" on your Button, that'll make it work like a LinkButton.

Here's my complete test code. Being C# I had to make a few changes as it doesn't have Handles - which maybe a key to the issue as C# and VB handle events slightly differently

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Test3.aspx.cs" Inherits="Test3" %>

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
        <div>

            <asp:ScriptManager ID="PageScriptManager" runat="server" />
            <asp:UpdatePanel ID="upPan" runat="server" UpdateMode="Conditional">
                <ContentTemplate>
                    <asp:Button ID="btnSave" ClientIDMode="Static" runat="Server" Text="Button" CausesValidation="false" OnClick="btnSave_Click" />
                    <asp:LinkButton ID="lnkButton" ClientIDMode="Static" runat="Server" Text="Link Button" CausesValidation="false" OnClick="btnSave_Click" />
                    <asp:TextBox ID="txtBox" runat="server" TextMode="MultiLine" Rows="3" />
                </ContentTemplate>
            </asp:UpdatePanel>

        </div>
    </form>
</body>
</html>

CodeBehind:

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public partial class Test : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        txtBox.Text = "Page_Loaded";
    }

    protected void btnSave_Click(object sender, EventArgs e)
    {
        txtBox.Text += "\n" + DateTime.Now.ToString("mm:ss:fff");
    }

    protected void Page_PreRenderComplete(object sender, EventArgs e)
    {
        txtBox.Text += "\nPreRenderComplete";
    }
}

Clicking the Button (or the LinkButton) works and updates the TextBox everytime:

Page_Loaded
55:54:185
PreRenderComplete