Thank you very much for the additional information.
For performance reasons, there is an upper limit for the number of elements displayed in yEd's context views. By default, this limit is set to 50 nodes and 100 edges. In your case, the node count of the neighborhood of the selected node exceeds the default limit.
You can change the aforementioned limit in "File" -> "Preferences", tab "Display", options "Maximum Number of Nodes in Context Views" and "Maximum Number of Edges in Context Views".