If you add items to the minecraft:villager_plantable_seeds tag, the expectation is Farm Villagers will be able to pick up the new items to put into the inventory so they can plant them onto farmland block later. Using the attached DandelionPoppyVillager.zip datapack, it tags Dandelions and Poppy as plantable by the Farmer Villager. However, dropping those items around shows the Farmer will not pick up the items. And thus, never will have the item in their inventory to plant down on Farmland blocks