Hope it will be manageable with pictures).
For example from these, if I want to produce 1(one) freshly developed mace without stopping current bootleg business, I should precisely:
1) click on grog item
2) click rmb on left down button (to manually drive away all those reluctant workers - shoo, shoo!)
3) close the box
Only then I can assign now "available" workers to new project. May be it look as not a very big deal (but how about case with multiple running projects?), but considering miriads of possible items in megamods, it should be handy.
Especially for new players, who had no idea about time and resources required for manufacturing of newly developed item so usually need to open "new production" box 2 times - first for examine requirements in detail and second for actual start of producing.