Removing a Lesson from the library also deletes it from within any Playlist I’ve added it to previously. After I do a lesson, I add it to a Playlist if I want to retain it for additional use later. Then I remove it from the Library page so the Library only shows lessons I haven’t seen yet. I just discovered several lessons within Playlists are no longer there because of this.
I noticed this does not happen to Lessons within a Course or a Course itself. If I add a Course to a Playlist and delete either the entire Course or an individual Lesson in the Library view the content does not get removed from the Playlist.