When farmers pick up plantable items, they absorb an entire stack of the plantable item to replace a single item from their inventory.

Example: A farmer with a full inventory including at least 1 stack of 64 potatoes harvests a potato block and plants a new potato in its place. The harvest produces 2-5 potatoes. The farmer picks up all of those potatoes. It replaces the 1 planted from its inventory and consumes/absorbs/deletes the rest.

Steps to reproduce

1. High in the sky, /setblock ~~~ dirt.
2. A few blocks away set up a water block so that when the dirt is tilled, the farmland will stay hydrated.
3. Spawn a villager on the dirt block.
4. Use a bed an composter nearby to turn the villager into a farmer.
5. Toss the villager 9 stacks of potatoes, carrots, or wheat/beetroot seeds. Observe that it picks up 8 of the stacks and leaves one floating.
6. Till the dirt with a hoe.

Expected result

The farmer plants a potato and then picks up one potato from the floating stack to replace the one planted from its inventory.

Actual result

The farmer plants a potato and then picks up the entire stack of 64 potatoes, replacing one in its inventory and consuming the other 63.

Impact

In 1.16 farmers were given the ability to continue harvesting when their inventories are full. This feature is only partly functional due to this bug. Farmers leave wheat and beetroot floating on the ground where the player or a hopper minecart underneath can collect it, but they absorb all excess seeds, carrots, and potatoes.