Staging sub-inventory during Pick Release

Hi,

We are on R12.1.3

All Release rules defined in the system have "Staging" (ST) as the default staging sub-inventory under the Inventory tab. Recently a shipper Pick Released an order without specifying any Release Rule. The order got pick released but it was noticed that the system moved goods from "Finished Goods" sub-inventory (FG) to a sub-inventory named "AC". The shipper has mentioned that he did not modify anything while Pick Releasing the order. Just entered the order number and clicked on Concurrent button.

We are struggling to understand how system moved goods from FG to AC. Ideally it should have moved goods from FG to ST. Please advise if there is a way to find this out.