The appearance of arrows where the tree cannot be expanded is indeed arguably a bug, but this behaviour represents a compromise for performance reasons.
Building the tree structure on a user's screen takes time. So that the user does not have to wait for the whole tree to load, only two levels are retrieved from the server at a time. When a white arrowhead is clicked, the node is opened if it has been downloaded already; otherwise the next two levels are retrieved from the server. From the user's perspective this amounts to checking to see whether there are child records or not. When a white arrowhead is clicked and no additional levels are available for retrieval, it will disappear.