We're seeing the same thing here. I'm a Canvas Admin here at my school. I have an instructor that complained about her quizzes getting mixed up also. I can find no rhyme or reason to the ordering. I then played around in my sandbox course. I have 2 quizzes:
Fire Safety Quiz due Feb 27
Color Quiz due Feb 20
The Quizzes page is displaying the Fire Safety Quiz first. The Color Quiz should be first whether you order alphabetically or by due date. So something is off. And my instructor has 24 quizzes that are all jumbled up together.