Landed cost items should update the item costs when a freight or duty voucher is posted. For standard costed items the actual cost is updated and normal item costing processes will apply where a company can subsequently update standard costs from actual based on their operational processes. Average costed items by their nature must be updated immediately. As such the item site value of the item is updated, thereby updating the average cost of the item immediately.
If you are not seeing this behaviour, then I suggest you raise an bug issue with some steps to reproduce the problem you are experiencing.