Fulfillment notifications are not being sent for child multi-address orders.

Fulfillment notification delivery is actually handled by whatever service you are using to fulfill orders. If this is a third […]

December 30, 2021

Fulfillment notification delivery is actually handled by whatever service you are using to fulfill orders. If this is a third party application, this application will need to explicitly set the “notify_customer” property to “true”. If fulfilling in Shopify, you will need to explicitly check the box which states that a fulfillment notification should be sent.

https://shopify.dev/api/admin-rest/2021-10/resources/fulfillment#[post]/admin/api/2021-10/fulfillments.json
Zac Fair