Villagers will only pick up 4 stacks if thrown to them or even if dispensed from dropper. This leaves 4 empty slots that they can fill with other things. This breaks Automatic wheat farms and Beetroot farms as they can still pick up other items just not any more seeds. If you throw 4 stacks of one item like seeds they can then pick up another 4 stacks of some other item but not the same seed. They also will not pick up items of the same 4 stacks if they harvest a crop.