Add custom attributes when creating product via REST API

Question

I am attempting to set custom attributes when creating a new product via the REST API. The products have a required attribute hpUuid. This is part of my JSON file to create the new product:

{
    "product": {
        "sku": "12345",

        ... Quite a few fields here ...

        "custom_attributes": [
            {
                "attribute_code": "hpUuid",
                "value": "123"
            }
        ],
    }
}

When submitting, the REST API returns this error:

{"message":"The \"hpUuid\" attribute value is empty. Set the attribute and try again."}

The fine docs suggest that the format is correct, but why can it not read the value?

EDIT: More information

I have tried two similar request bodies, based on different examples that I have found. One based on the fine manual is here:

{
    "product": {
        "name": "My Fine Product",
        "sku": "123",
        "attribute_set_id": 4,
        "status": 1,
        "visibility": 4,
        "type_id": "configurable",
        "extension_attributes": {
            "category_links": [
                {
                    "position": 0,
                    "category_id": "2"
                }
            ]
        },
        "custom_attributes": [
            {
                "attribute_code": "hpUuid",
                "value": "52"
            }
        ]
    }
}

And another based on an accepted Stack Overflow answer is here:

{
    "product": {
        "sku": "sku_from_rest",
        "name": "My Fine Product",
        "attribute_set_id": "4",
        "status": 1,
        "visibility": 4,
        "type_id": "virtual",
        "weight": 0,
        "extension_attributes": {
            "stock_item": {
                "stock_id": 1,
                "qty": 20,
                "is_in_stock": true,
                "is_qty_decimal": false,
                "use_config_min_qty": true,
                "min_qty": 0,
                "use_config_min_sale_qty": 0,
                "min_sale_qty": 0,
                "use_config_max_sale_qty": true,
                "max_sale_qty": 0,
                "use_config_backorders": false,
                "backorders": 0,
                "use_config_notify_stock_qty": true,
                "notify_stock_qty": 20,
                "use_config_qty_increments": false,
                "qty_increments": 0,
                "use_config_enable_qty_inc": false,
                "enable_qty_increments": false,
                "use_config_manage_stock": true,
                "manage_stock": true,
                "low_stock_date": "string",
                "is_decimal_divided": true,
                "stock_status_changed_auto": 0,
                "extension_attributes": {}
            }
        },
        "custom_attributes": [
            {
                "attribute_code": "hpUuid",
                "value": "123"
            }
        ]
    }
}

I have ensured that the attribute_set_id value of 4 is correct for the default attribute set. I have addionally added an attribute set number 9 which is a clone of the Default set. However, when I use the attribute_set_id value of 9 the product is created without an hpUuid, as if the value that I submitted was simply ignored.