When making a partial PostBack while the Combos are inside a RadAjaxPanel, the components initialize in the reverse order, where the last Combo initializes first, the second to last initializes next, and so on. This way, the Child combos can't access the Parent's values as the parent is not initialized at that time.

I have a RadAjaxManagerProxy in a user control that settings are added to on Page_Load:

If Me.Visible AndAlso rampServiceCharges.AjaxSettings.Count = 0 Then
    rampServiceCharges.AjaxSettings.AddAjaxSetting(rampServiceCharges, gvCharges, ralpServiceChargesDefaultSkin)
    rampServiceCharges.AjaxSettings.AddAjaxSetting(gvCharges, gvCharges, ralpServiceChargesDefaultSkin)
    rampServiceCharges.AjaxSettings.AddAjaxSetting(gvCharges, ddlFilterStatus, Nothing, UpdatePanelRenderMode.Inline)
    rampServiceCharges.AjaxSettings.AddAjaxSetting(ddlFilterStatus, gvCharges, ralpServiceChargesDefaultSkin)
End If

These settings had to be moved from the front end to the code behind because it was causing issues when the user control isn't visible. In the front end code, it is possible to assign a CSS class to an UpdatePanel, but there appears to be no such overload when adding AjaxSettings programmatically.

<telerik:AjaxUpdatedControl ControlID="Button1" UpdatePanelCssClass="updatePanelClassName" />

https://docs.telerik.com/devtools/aspnet-ajax/controls/ajaxmanager/how-to/add-ajaxsettings-programmatically