Issues Creating Fulfillments and Fulfillment Events via Admin API

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I’m working on an integration using the Admin API to create fulfillments and then update their status via fulfillment events.

The flow works perfectly in our test production store: https://mystore.myshopify.com. Here’s a summary:

  • Create Fulfillment:
    POST /admin/api/2025-04/fulfillments.json
    BODY:
    {
    “fulfillment”: {
    “message”: “The package was shipped this morning.”,
    “notify_customer”: false,
    “tracking_info”: {
    “number”: “1562678”,
    “url”: “https://www.my-shipping-company.com”,
    “company”: “my-shipping-company”
    },
    “line_items_by_fulfillment_order”: [
    {
    “fulfillment_order_id”: {fulfillment_order_id}
    }
    ]
    }
    }

*Add Fulfillment Event:
POST /admin/api/2024-07/orders/{order_id}/fulfillments/{fulfillment_id}/events.json`
BODY: { “event”: { “status”: “in_transit” } }

However, in two client stores using the exact same app and API scopes, the API does not create the fulfillment, 404 Not found responses. And even if I manually prepare the order in the Admin UI, trying to create a fulfillment event via API returns an empty response.
All permissions (scopes) of the app associated with the token are enabled, so we’ve ruled out a missing scope as the cause.

Is there anything specific that could cause this difference in behavior between stores?