It's an RM decision. This is when ExpertFlyer fails; AC RM can factor the existing itinerary into availability. Those of you searching individual flights are not getting the full picture.
Here's AC1847 availability, searching on it's own:
1 *AC1847 J9 C9 D9 Z9 P7 R0 Y9 B9 M9 U9 H9 Q9 /PVR YYZ 1 145P 715P E0/763 4:30
Here's AC1847 availability, searching when already holding 1 P class seat on AC1846 on 31DEC;
1 *AC1847 J9 C9 D9 Z6 P0 R0 Y9 B9 M9 U9 /PVR YYZ 1 145P 715P E0/763 4:30
P is not just open or closed; it is more complicated than that.
bawm