Not sure if that`s a bug or not, but since the Farming skill description says:
"You know how to till and plow, and how to plant seeds."
I assume we were meant to be able to plant seeds only AFTER acquiring this skill.
I tried planting a poppy seed by left clicking to select the seeds, and then right clicking on a plowed tile. No success.
Then I shift + left clicked the seeds, to get the "planting" cursor, selected a plowed tile and was able to plant.