Why System Traceability Report not work for library blocks?

I have a suite of library blocks my organization has created, and underneath the mask of each library the blocks within have traceability links to the requirements defining the libraries.

I understand that when viewing the library block by itself (LibraryX.slx), there are no reporting options available ("Reports" not visible under the File menu). However, if I place the library block in a model (such as a Test_Harness.slx), and navigate to File -> Reports -> System Requirements Traceability, I receive an error message that the model has no links stored in a corresponding .req file. This kind of makes sense, because there is no Test_Harness.req file, but I would think the reporting tool could traverse the library link and discover there is a LibraryX.req file.

On the other hand, if I select File -> Reports -> System Design Description then the resulting report does contain the traceability of the library, so this type of report is able to follow the library link and discover the .req file.

Behavior noticed in Simulink R2016b with traceability to a Word document.