Exporting lessons by student only exports some lessons

[donnapep]

Steps to Reproduce

1. Create a course with at least 10 lessons. (Simplest thing would be to import and publish the demo course.)
2. As a student, start taking the course.
3. Browse to wp-admin/admin.php?page=sensei_reports&user_id=<user_id>&course_id=<course_id> for the given user and course ID.
4. Export all rows.

What I Expected

All rows to be exported.

What Happened Instead

Only some rows are exported; in my case, 6 out of 12 rows were exported.

Context / Source

Reported by Gonzalo in email. We should check whether other exports have this issue as well.