When an order has been sent to Shopify and is marked as fulfilled the fulfillment information including tracking data automatically flows back to the marketplace and updates the shipment status of the order.
For eBay orders, the fulfillment information is part of the feedback system, so turning the eBay feedback to Never, will disable the fulfilment updates. If you want Marketplace Connect to send fulfillment information but not feedback messages to buyers, simply clear the list of feedback messages.
For further assistance, please reach out to marketplace-connect@shopify.com