To reproduce this:

1. Set the GroupRenderMode of RadGridView to Flat.
   
2. Page RadGridView using RadDataPager.
   
3. Scroll to the last row in the view. Or scroll to any row so that few of the rows above are not longer visible.
   
4. Expand the row details and scroll down more until only the parent row is visible.
   
5. Collapse the row details. At this point, the vertical scrollbar disappears and the upper rows are not brought into the view as expected when the row details gets collapsed.

To work this around call the Rebind() method of RadGridView on RowDetailsVisibilityChanged event.

private void RadGridView_RowDetailsVisibilityChanged(object sender, Telerik.Windows.Controls.GridView.GridViewRowDetailsEventArgs e)
{
	if (e.Visibility == Visibility.Collapsed)
	{
		this.gridView.Rebind();
	}        
}