When cancelling a Posted Purchase Invoice. The connected posted shipment doesn't indicate that it is cancelled. Therefore you cannot see whether or not the posted shipments actually have impact on the inventory, when looking from the posted shipments page alone.

Solution 1, indicate on the posted purchase receipt that it is cancelled.

Another solution, could be to post a purchase return shipments in combination with the cancellation of the purchase invoice.. Logically this would properly make sense, since the cancelling the purchase invoice for an item will have effect on the inventory. So for me it is weird this is not displayed as a return.