bentbanana
Do I understand correctly that the A320 has 3 cabin zones and the A321 has 4 cabin zones?
This being the case, the late reallocation of an A320 load to an A321 would have resulted in Zone D being empty. The 42 pax shown in Zone D on the loadsheet would have previously been seated across Zones A, B & C on the A320. During the change of gauge to A321 there must have been 44 pax moved into Zone D to achieve trim. Evidently these pax were not issued with new boarding passes and a communications breakdown has resulted in their remaining in Zones A, B & C. With additional seats in the first three zones of the A321 there were probably no seat dupes to raise alarm and if there were they were probably just told to sit in the vacant seats in Zones A, B & C. What should have raised alarm was the vacant Zone D but the cabin crew probably assumed that it was deliberate for trim.