I created quiz Lesson with 27 questions in it. Then I tried to change randomization settings to "Categorized random questions". Then changed it back to "Categorized random questions". And now there is now questions associated with Lesson. And only 16 questions are available to include in the Lesson. I fear that with large number of questions and it would be difficult to search for them to add back to the lesson.
After editing of the single
Thank you, James!