Questions lot number not assigned during receipt

I'm working on an implementation (2019R1) where client requires specific cost, and we want to assign a unique lot number to each item on a receipt - e.g 5000 pounds of item FAIRYDUST dust is assigned lot # 1, while 4500 liters of POTION is assigned lot #2, etc

I've set up a lot/serial class and an item as shown in screen shots below.   no matter what I do, all items received (using either Purchase Receipts or Inventory Receipts) are assigned a lot number of 0.   before releasing the receipt, if I open the Allocations tab, clear the row with the assigned lot number of 0, and click Generate, it still assigns a lot number of 0.

any suggestions would be most appreciated.

thanks

   jim