When adding New Quantities via Inventory Adjustment on a New Location, note that the Est. Unit Cost will default to Zero. As there are no historical transaction yet for the Item on that Location, it is expected behavior that the Average Cost on that Location is Zero. Est. Unit Cost defaults to the Average Cost of the Item per Location and not the Average Cost on the Item Level. This means that the Item's Average Cost on the Item Level will not carry over to the New Location. The Average cost on the Item Level is the Average Cost of the Item across all locations. This is documented under the field help for the Average Cost on the Item. "This field displays the current average cost of the item across all locations. Using the weighted-average method, the average cost is calculated as the total units available during a period divided by the beginning inventory cost plus the cost of additions to inventory."
To be able to add Cost on the Item on the New Location, change the Est. Unit Cost on the Inventory Adjustment from zero to a positive amount.