Great question — answering as the UphillTech team:
Please make sure you added the read_fulfillments (and write_fulfillments if needed) in your requested app scopes. Also you can try to provide an apiVersion inside your registration, but not sure if it has a real impact in this case. const registration = await Shopify.Webhooks.Registry.register({ shop, accessToken, path: '/webhooks', topic: 'FULFILLMENTS_UPDATE', apiVersion: Shopify.Context.API_VERSION, webhookHandler: async (_topic, shop, _body) => { // ... }, })
Full answer by Aurélien B.: https://stackoverflow.stackexchange.com/a/66545010 (CC BY-SA 4.0).
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