How to troubleshoot WMS FIFO (First-In, First-Out) failure regarding bin locations and picking logic?

Today, a food SKU was returned by a customer, who claimed the batch received was even older than the one from last week. Our WMS clearly has FIFO enabled, and the picker followed the system tasks, so the issue wasn't immediately apparent. I first checked the lot master and found the batch dates were correct. Then, I looked at the bin location attributes and realized that when the forward pick…