OpenNodes in dataprovider of Advanced Data Grid is empty

Question

I am having a hierarchical ArrayCollection and set it as a dataprovider to an ADG

<mx:AdvancedDataGrid>
  <mx:dataProvider>
    <mx:HierarchicalData source="{arrCol_groupedData}"
    childrenField="accounts" id="hierarchicalData"/>
  </mx:dataProvider>
<mx:AdvancedDataGrid>

Why is the openNodes array always empty? (I need it to preserve the open nodes after resfreshing the data)

It worked fine with a GroupingCollection, when Flex made the grouping out of a flat data.

Any ideas?

Thx, martin