In Increase Line Quantity screen, value for "Available Qty" column is
not displayed if the inventory is available.
To fetch the availability details of the item, getFulfillmentOptionsForLines API gets called on the Increase Line Quantity screen. This API does not return the total available qty of the item in the system.
The column "Available Qty" is not meant to show the total Available Qty for the item. It only provides the qty available for the line. When the qty requested exceeds the available qty of the item, it displays the maximum qty that is available for the line. In the scenario when the qty is available the product does not show this explicitly as this information is already available in the form of (Ordered Qty + Additional Qty).