The only thing I have heard was how pleased the airline was with their fuel efficiency.
I think Air Canada is delaying the 7M9 as they have sufficient capacity with their 7M8 and A321 fleet. The A321 should not need replacement for another decade. Why add a variant if they don’t really need it? In 5-10 years, Air Canada can choose between the A321neo, Max9, Max10, or 797.

Don’t forget, the 7M8 already provides the airline a capacity increase vs the A320. As they still operate a fleet of A319 and A321 aircraft (and eventually A320s) at Rouge, operating a sub-fleet of A321ceo at AC mainline should not be an issue.