Rest API Controller does not fetch data for meta key properly

Thanks a lot for your REST API Controller plugin which has helped me give a client access to my content via API.

I have a little problem though. I use Sensei LMS and there is a meta key (_question_order) which normally displays a list of question IDs related to a particular quiz. The problem is that this meta key works well and displays the question IDs for a few quizzes but not all. In fact over 95 percent of quizzes do not display any data under _question_order and I can’t seem to find what the problem is because all the quizzes have the same settings and all have questions and are supposed to display the questions ID under _question_order.

Why are only a handful displaying data for this meta key?

Please, your help will be highly appreciated.