CP and AC can't just dip into each other's inventory. This is a code-share relationship, not joint marketing.
Since CP and AC codeshare on each other's flights, this means that each carrier sells the other a block of seats for a fixed price. The buying carrier then has to sell them, or take the loss, as the case may be.
All that your experience means is that CP used up all of the seats they bought from AC on this flight. AC might sell them more, if their direct sales do not move and CP is willing to buy them.