Unless they have changes something in the game mechanics, they gave a clear explaination on how the explorers worked several years ago. The loot for the explorer is determined at the time the explorer is sent out. If the explorer is sent out before the phase 1 ends, the loot will include pumpkins. However, the explorer loot is not saved when your island goes into hibernation 15 minutes after you log out. When you log back in and the island is updated for the time you were off-line (up to 24 hours), the explorer loot is also redone. If this happens after phase 1 ends, it will use the active loot table which does not include pumpkins.
You can maintain the event loot for the explorer if you do not log out and reset the loot tables. This has worked in the previous events and has worked so far in this event with my explorer who have returned after the event ended with pumpkins.
Here are the instructions on how to maximize the event returns with explorers.