The bug

Pumpkin-equipped mobs spawning during Halloween never drop their pumpkins when killed. 

The problem is that their ArmorDropChances is being set to 0 instead of 0.085.

Reproduction steps:

1. Set your computer time to Oct 31.
2. Start a new MC world
3. Give yourself a weapon with looting, the higher the looting is, the better
4. Set the time to night
5. Find some mobs with pumpkin on their heads
6. Kill them
7. → No pumpkins
   Note: If you stick to the maximum level of looting (3), you might need to kill many to actually reproduce this and to make you not think that you had bad luck.