A farmer villager will try to pick up items when its inventory is full instead of continuing to harvest crops (1.15.2 and 20w13a) or use the composer in the new update(20w13a).

I would expect that a villager would run a check of its inventory before attempting to pick up an item.

What happens is that a farmer villager with a full inventory will walk over to an item, attempt to pick it up, wonder away, repeat, until (I assume) the unlikely event that it randomly wonders out of its range to detect items it wants to pick up.  This tends to occur most when the crops around the villager are all planted with a crop. This happens in 1.15.2 and the latest snapshot (20w13a).

Specifically to the new snapshot (20w13a) the farmer villager will prioritize picking up items it can't over harvesting and planting a new crop and possibly creating space in its inventory by using the composer.

This creates issues with villager crop farms in the past (1.15.2 and I assume some previous versions) (mostly issues with wheat/bread and beetroot farms) because the villager will spend time trying to pick up items when it could be harvesting plants and leaving the seeds to despawn.

Will happen with all items the farmer villager will attempt to pick up, wheat, wheat seeds, bone-meal, beetroot, beetroot seeds, etc.

The farmer villager will do this repeated attempt to pick up items if their inventory is only full of the item it is trying to pick up - for example, the villager has 4 stacks of beetroot seeds and empty slots for other items in its inventory, then repeatedly tries to pick up more beetroot seeds - or it its inventory is completely full - for example, the villager has 4 stacks of beetroot seeds and 4 stacks of beets, then repeatedly tries to pick up bone-meal.

Ways to fix the issue :

• add a check of inventory prior to attempting to pick up items to determine if it is possible to pick up the items (because its inventory might be full of that item or other items)
• have the villager prioritize farming and/or composting and/or dispersing items to other villagers over picking up items 
• allow the villager to compost extra beets, wheat, carrots, and potatoes (though this might impact the player giving food to a villager breeder because the player might want the villager to collect all 4 stacks of food for breeding instead of having some of it go to waste in the composer, so not the ideal way to fix)

Visual example : https://youtu.be/P-gE6JZzYAs