Thank you for providing the code.
I managed to reproduce the problem locally. It turned out that this is a bug with the current version of RadTreeView "Prometheus". The state of the first added parent node is not persisted.
One thing you could do is to set the ExpandMode of this particular node to ClientSide instead of ServerSideCallback, since its nodes would have been loaded already. This, however, will affect the node's state only visually.
I have logged this bug to our bug-tracking system. It should be fixed prior to our next official release.
Your Telerik points have been updated for the report.
We apologize for the caused inconvenience.
the Telerik team