I've had a look but can't seem to find anything that addresses this error, so hopefully someone here can help.
Some of our quizzes include essay questions, and when the student is required to answer the question with things that require special formatting (tables, equations, diagrams, etc.), we give them the option of uploading a file instead of answering in the text box. File uploads are set as optional, and restricted to a single upload.
Our markers have found that if a student answers these essay type questions with only a file upload, and does not edit the text box, then their answer doesn't get passed through correctly for marking. On the manual grading page, these questions do not appear in the 'To grade' column, and can only be viewed when 'grade all' is selected in the 'Total' column. You can see in this screen shot how the totals do not add up for any of these questions.
We're using Moodle 3.1.7+.
Does this still happen in the latest version of Moodle? Can you try on qa.moodle.net?
I assume that, when you review the attempt, it also says that the state of the question is 'Not answered'?
If this is still broken in the latest version then it is probably a minor bug in the logic in the is_complete_response method in question/type/essay/question.php