I had placed several chests made of hay and the hearthlings started running in place in front of the chests. They also kept lifting and dropping things around the chests instead of into them, and lastly they all began to sit in/on the chests while eating.
The whole game started acting weirdly as soon as I had put the chests to use.
Furthermore, most of them all keep standing in the exact same spot next to the chest, until I give them something to do. As soon as they have done the task they run back to the same spot.
After deleting all the chests the hearthlings all act normal again