We found Unit Cost of Bill's items are copying from Purchase Order, not from Purchase Receipt now.
That is weird, because it copied Unit Cost from Purchase Receipt in Acumatica 2018R1. And sometimes vendors might have special deal when ship out so we will change the Unit Cost on Purchase Receipt.
And consulted with Acumatica Support, support told us need to change the Unit Cost on Purchase Order. However, when we found the Unit Cost is not consistent with PO Receipt, we couldn't modify Purchase Order anymore.
Another funny thing is we can click "Add PO Receipt" on Document Details, but the data is not coming from PO Receipt. That is very ridiculous.
The function worked very well before 2018R1 but suddenly been "changed" when we upgraded to 2019R1. Hope Acumatica can add back the function or at least provide option to let customer to do that.