Whenever you press the use button to equip a helmet, it takes whatever item off your head and in your hand. If you equip a helmet with the use button while wearing a pumpkin, it replaces the pumpkin and places it on the ground.
MCPE-173875
Replacing a Carved Pumpkin on your head with a Helmet places the Pumpkin on the ground
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Does this only happen when your inventory is full?
And do you mean dropping the Jack o’ Lantern item, or placing it as a block?