I cannot reproduce the issue at my end. Check the following demo which implements the same. Grid / Programmatic Binding
Please elaborate your scenario if it doesn't help.
I think I was having the same problem you were. I was using a window to edit a record in a detail table, and wanted to rebind just the detail table after the user clicked Save. Turns out you have to rebind the entire grid, then re-expand the detail tables. Here is how I did it.
Protected Sub RadAjaxManager1_AjaxRequest(ByVal sender As Object, _
ByVal e As AjaxRequestEventArgs) Handles RadAjaxManager1.AjaxRequest
If e.Argument = "Rebind" Then
grdTasks.MasterTableView.SortExpressions.Clear()
Dim expandedItems As String = ""
For Each gi As GridItem In grdTasks.MasterTableView.Items