Cost Add On Question

We have some items that are manufactured (M BOM and Routing type) that we also occasionally sent to an outside processor (A1 Routing type).  We also use a Cost Add On X3 based on our B1 Direct Labor cost.  When Sending to an outside processor, the WO has no actual or planned B1 cost, but it still has the X3 cost.  With no direct labor, I would expect the actual X3 cost to be 0 for the WO, but it is still adding the standard value for X3.  Is this correct and my assumption wrong, or is this a bug? 

Thanks,