Root Cause
This is working as per iVend system design. The weighed product quantity prompt behaviour at POS is governed by the current stock level of the item in the store database:
Stock Quantity POS Behaviour
Stock = 0 or less than 1 Quantity prompt does NOT appear — system uses the default/stock quantity
Stock = 1 or greater than 1 Quantity prompt DOES appear — cashier can enter the weighed quantity
When the store's inventory for a weighed product shows a stock quantity of less than 1 (e.g., 0, 0.5, or negative), the POS engine does not trigger the quantity input prompt. This is a known design constraint in iVend's weighed product handling logic.