Hi,
While creating manufacturing order there will be a stock move , the components move from WH/Stock (your main warehouse- internal location) to Production location (production), so that there is decreasing in the components quantity, according to the product we want to produce, the finished goods move the production location to corresponding internal location that means the WH/Stock (your main warehouse) then the quantity will updated , If the location is not set properly there is no change in the onhand quantity, please make sure that you set the location properly.
Regards