The query looks good at a glance, so I would suppose that there’s another component of the order here that’s preventing the mutation from reducing the quantity of the item. Are you able to run the query against your shop on orders that have different fulfillment statuses for the line item you are trying to modify? My first assumption would be that there’s a fulfillment state preventing a quantity reduction, but I’m running the logs at the moment to see what else might be happening.