It's not just my plugin, anyway.
The Favorites Menu plugin behaves the same way: works from the main menu, but not from the library's context menu (on Arch Linux). Right-clicking a library entry after first launching calibre, will result in an empty menu for the plugin.
Seems the last menu built "on the fly" from the main toolbar will persist when next attempting to use the context menu, but the aboutToShow signal is not firing on the context menu. This is not tragic for the Favorites Menu plugin (since once the menu is configured, the menu doesn't really change often). However for my plugin, the last menu built from the main toolbar will often contain actions set to operate on a different book than the one that is currently highlighted.