Hello again Stephan,
This is a known issue that has already been fixed in the latest internal builds and in the beta release, and the changes will be available in the next official release. The problem can be workaround with the current version by setting the "data-expanded" attribute on the nodes e.g.
.ItemDataBound((item, customer) =>
item.Text = customer.ContactName;
item.Id = customer.CustomerID;
item.Expanded = ((
the Telerik team
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