Complete fulfilment orders for different warehouses

Retired Boards Appdev Fulfillment Api
2

So there are two fulfilment orders:

{
  "fulfillment_orders": [{
    "id": 4982117007546,
    "order_id": 4053182349498,
    "assigned_location_id": 62918459578,
    "request_status": "unsubmitted",
    ...
    "line_items": [{
      "id": 10494937170106,
      "fulfillment_order_id": 4982117007546,
      "quantity": 1,
      "line_item_id": 10380418908346,
      "inventory_item_id": 40166440599738,
      "fulfillable_quantity": 1,
      "variant_id": 38072997675194
    }],
    and another one...
  }]
}

Sending a POST request without any line_items results in:

"fulfillment": {
      "tracking_company": CARRIER_NAME
      "location_id": LOCATION_ID,
      "tracking_number": BARCODE,
      "tracking_urls": [TRACKINGURL],
      "notify_customer": true
    }

Results in “None of the items are stocked at the new location.”
Sending the same request including line items. Results in “Not Found”.

If we put both products in the same warehouse, we then get 1 fulfilment order. And it is possible for me then to partially ship the order. With the same requests…

So what is going on here Shopify?
Regards, Peter