Thanks for your reply Allie,

I figured out eventually what the problem was, and it was the result of a strange coincidence of errors:

1. The posting policy wasn't set to manual after all. Not sure how this happened since I knew to do that and remember doing it when I set up the assignments a week before this happened. Perhaps my changes in the settings weren't properly registered due to a connectivity issue or something of that sort. Either that, or Canvas has grown sentient and wants to mess with me! 
2. So if it wasn't set to "manual", why were most students' grades hidden anyway? In the quiz, I gave students an option to skip one question of their choosing. As I graded, I simply skipped those questions as well.  As one question per assignment was not graded, Canvas considered the grading as incomplete and did not post the tally in the Grades module. 
3. So why was the grade posted for one student only? Because they did not read the instructions properly and answered all questions. I did not realise this and went ahead and graded all questions as well. So for this one single student, Canvas considered the grading complete and posted the tally in Grades. 

In the end, I had to go back to every quiz and manually enter a grade of 0 for the skipped question. Only then did Canvas post the grades. 

I know I can hide grades after the fact, but the student had already seen the grade. I would not have known about the error had the student not written to me about their grade. In any case, it's all been sorted now. However, I am still concerned about the manual setting not registering or getting reversed after the fact. I guess I'll have to double and triple-check in the future.