Thanks Sean and 24Left for sharing your experiences.
It is my first time using the business class service for Air Canada. Last flight was Premium Economy for the same flight during this past Christmas.
I didn't have this problem at that time.
I usually fly Korean Airlines and never heard of having to monitor my seat assignment for random changes. I guess now I know that I need to constantly monitor.
I just thought they will take care of you when you buy their premium product.