WooCommerce ACF not updating with JSON

I cant’t UPDATE or POST the customs fields on my WooCommerce products, The options show in rest and edit in rest are enabled and I can see the fields in the GET request with Postman, but when I try to update them, they are ignored.

I update with this JSON format:

{
    "fields": {
        "pack_quantity": "22",    
        "continue_selling_out_of_stock": false
    }
}

But they don’t change (with PUT or POST), they continue as NULL:

"acf": {
         "pack_quantity": null,
         "continue_selling_out_of_stock": null
        }

Am I missing anything to update or post a custom field for a product?

Any help on this would be much appreciated.

Thanks