What I have lately noticed is that they actually do finish it but it looks like they were stuck. What I mean is that the circle for the action looks full like in your screenshot but if you then open the inventory and hover over the piece they are producing you see that it is only at 50% when the circle is full, so it actually takes them twice the time to really finish the piece. For programming it is nearly 4 hours and when the piece is at 100% they actually do finish the action and stand up. Could this be the case in your game as well @Samuriko ?
Nevertheless it is a bug either way as it is very misleading. The circle should only be full when the piece they are producing is at 100% and not 2 hours before it is the case.