I found this error when I created a test course. There I added some mcq elements along with assessments (like answer field, file upload,..) that have to be graded manually by the instructor. The error is, the course gets mark as completed and the certificate gets generated without instructor grading for manually graded assessment elements (the unit shows it is 100% completed). The plugin don't take manually graded assessment elements as a criteria to determine whether a student completed the course or not. It only considers automated MCQ elements. Even in MCQ's I found that even though a answer is wrong for a question the unit gets mark as completed and thus the course.
Can someone please try to replicate this on your own Coursepress installation and let me know if this is a bug or not?