Unexpected Behavior with Standard Cost

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Dear Experts

There are three instances brought to attention by end user after two months. Published Standard Cost was there the system. But when cost processors runs then it does not find the standard cost of Purchase Order Receipt and comes as error as "The receipt is missing a cost".

User saw that message and gone ahead and defined the cost and published a new cost. (Different than what was originally)

During the internal audit, after 2 months we are finding that standard cost was there in the system when PO Receipt transaction was came in.

Question is what would have caused for system not to see cost at that time. How come a cost could have disappeared while transaction was processing by cost processor?