This bug was first discovered by @aamyot
SolidusImporter::Processors::Shipment creates only one InventoryUnit per LineItem, when it should, in fact, create InventoryUnits equivalent to the LineItem amount.
Here's a suggestion:
def inventory_units
    sku = line_items_attributes[:sku]
    return [] if sku.blank?
    quantity = line_items_attributes[:quantity].to_i
    [{ sku: sku }] * quantity
  end