Maybe Fins 235–242 has their C/D check due very soon, they need to be taken out of service anyway, the CZ fins are needed to cover for them.

Or those CZ fins are short term (12-24 months) lease to bridge the gap between A321XLR and the additional MAX, they just don't bother.

Or AC is so short of planes this summer, they can't spare any fins for refit or maintenance especially how long it is taking to refit those fins.

I'm sure AC know what they are doing, and knowledge that is not available to us.
Putting the CZ fins directly into service is not ideal but probably more cost/time effective than parking them for another 2-6 months for cabin refit.