We have an Accounting Preference called Default Items to Zero Fulfilled/Received that can be found when you navigate to Setup > Accounting > Accounting Preferences > Order Management. It is possible that this preference is enabled in your account.
Check which suits your business process better. Do you usually need the Item Fulfillment and Item Receipt to be auto-populated? If yes, then make sure to clear this box. But if not, you can check this box to default items to unfulfilled/unreceived when you open a fulfillment/receipt transaction. Note that this preference is only for defaulting purposes and can be overridden in the Item Fulfillment and Item Receipt as needed.