If there are 25 widgets in a particular bin in an FC, I have serious doubts that the instruction and expectation of the picker is to pick the specific one that has this particular FNSKU.
If they want that behavior, it would seem far better (less error prone and faster) to segregate by FNSKU such that any one location has only one FNSKU for a given ASIN. (That they don’t do this [or didn’t always do this] seems to be the root of how you could get a counterfeit good from a “legit” seller, including Amazon.)
AFAIK they have always segregated different FNSKUs to separate bins for comingled inventory.
The reason you get counterfeit goods is that Amazon prefers to fulfill an order from warehouse A but seller 1 only has units in warehouse B. Seller 2 has units in warehouse A so Amazon performs an item swap between the sellers to save on shipping (or to improve shipping speed).
If they want that behavior, it would seem far better (less error prone and faster) to segregate by FNSKU such that any one location has only one FNSKU for a given ASIN. (That they don’t do this [or didn’t always do this] seems to be the root of how you could get a counterfeit good from a “legit” seller, including Amazon.)