"Failed to submit fulfill request" wen using the REST API to send a fulfillment request

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I'm trying to POST to this endpoint: https://shopify.dev/docs/admin-api/rest/reference/shipping-and-fulfillment/fulfillmentrequest?api[ve...

With this payload: 

  "fulfillment_request": {
    "message": "Please fulfill this.",
    "shipping_method": "Standard",
    "notify_customer": true
And the response is 422 'StatusError' with a message of 'Unprocessable Entity', and error text that says "Failed to submit fulfill request"

I'm confident that my fulfillment_order_id is correct (I can list the line items incorrectly and those get identified as missing, when I drop the line items as below or list them by correct ids then I see this error)

This is an Amazon Marketplace Web Fulfillment Request, and the shipping method does indeed match that shown in the GraphQL variable when requesting fulfillment through the UI

Here's the 'x-request-id' for a recent failed request: '2e4cf9e6-1910-4e08-9be6-c642b8ccf2bc'

Any help with this would be appreciated!