Book hidden chapters are not shown on WS

Log in as Admin. Create a Course. Enrol a Student in the Course. Enrol a Teacher in the Course. Create a Book activity in the Course. Go to the Book. Create several Chapters in the Book. Set one of the Chapters as hidden. Enable web services (Site administration > Advanced features). Create a tokens for the Student and the Teacher which are enrolled in the Book's course (Site administration > Plugins > Web services > Manage tokens). Call the core_course_get_contents web service as the Student (MOODLEINSTANCE/webservice/rest/server.php?wsfunction=core_course_get_contents&wstoken=XXXX&courseid=COURSEID) through a HTTP client for testing web services (ex. Postman) where: wstoken => the token created for the Student;  courseid => the course id; make sure the returned data does not include the hidden chapter. make sure "ishidden" attribute is present in the returned data for the Book chapter content ("ishidden" == false for each returned chapter). Call the core_course_get_contents web service as the Teacher (MOODLEINSTANCE/webservice/rest/server.php?wsfunction=core_course_get_contents&wstoken=XXXX&courseid=COURSEID) through a HTTP client for testing web services (ex. Postman) where: wstoken => the token created for the Teacher;  courseid => the course id; make sure the returned data does include the hidden chapter. make sure "ishidden" attribute is present in the returned data for the Book chapter content ("ishidden" == true for the hidden returned chapter,   "ishidden" == false for the rest of the returned chapters ).