Hi,

Similar features has WebAsyncRefreshPanel, so you may consider to use it instead of async options of tab.

It is also possible to work around that issue within events of UltraWebTab.
You may process ClientSideEvents.BeforeAsyncSubmit, check for trigger-control and raise full postback by oEvent.fullPostBack = true;
If you need to process event hadler on server for that trigger-control, then you need extra work. Below example will keep async postback for Button1 and trigger full postback for Button2. It also will "fake" call to the Button2_Click by setting a flag for one of the global fields on client and checking value of that field on server.
Codes in aspx:
<script type="text/javascript">
function UltraWebTab1_BeforeAsyncSubmit(oWebTab, notUsed, oEvent, id)
{
 if(id == '<%=Button2.ClientID%>')
 {
  oEvent.fullPostBack = true;
  var elem = document.getElementById('__EVENTTARGET');
  if(elem)
   elem.value = id;
 }
}
</script>
 <igtab:UltraWebTab ID="UltraWebTab1" runat="server" AsyncMode="On">
  <ClientSideEvents BeforeAsyncSubmit="UltraWebTab1_BeforeAsyncSubmit" />
  <Tabs>
   <igtab:Tab Text="Tab 1">
    <ContentTemplate>
     <asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
     <asp:Button ID="Button2" runat="server" Text="Button" onclick="Button2_Click" />
    </ContentTemplate>
   </igtab:Tab>
  </Tabs>
 </igtab:UltraWebTab>

Codes in cs:
protected void Page_Load(object sender, EventArgs e)
{
  if(this.IsPostBack)
  {
   string id = this.Request.Form["__EVENTTARGET"];
   if(!string.IsNullOrEmpty(id) && id.Equals(this.Button2.ClientID))
    this.Button2_Click(this.Button2, new EventArgs());
  }
}