This (while bad news) is incredibly helpful to know!
It means that I will NOT be switching to new quizzes for anything that contains essay questions. The reason I download them as a .csv is so that I can actually grade all of them at once (like in blackboard) where you can see all the entered answers to a single question at once and thus ensure better grading equity.
You can't upload those question-based grades back to Canvas, but as a work around, for short answer tests, I set up a formula in excel to match question numbers and grades into one cell and then paste that as a comment.
That's very disappointing to learn this was intended.