Hearthings cannot place flower-boxes without building entire scaffolding.
Hearthlings will not add flower-boxes to a building without adding the scaffolding back onto the structure, including the roof scaffolds.
Simply finish building a building, then attempt to place flower-boxes onto said building on the ground floor.
I expected no hassle, like a door or window. Simply placing down the item, however, the above result happened.
It took them two days to place the flowers. TWO. DAYS.
Version is develop-2391.