Dear Shopify Support Team,
I think I'm encountering this same issue.
On the Shopify Order's view you used to be able to see the tracking numbers for the shipments that had already shipped for a fulfillment. The order was still marked as "in progress" but that was expected since the fulfillment is not complete yet. However it would still show the tracking numbers that had been attached so far to the Fulfillment.
Now with what I assume is an issue with the new UI changes; the tracking numbers are not showing up until the very last item has been shipped for the Fulfillment. This means the fulfillment has to be completed before the tracking numbers are shown. This is not good. My Clients need to be able to see the tracking numbers already assigned to the Fulfillment. I double checked and made an API call to the order and it does return the Tracking numbers. So the data is available, but they are not being shown anymore. I need these tracking numbers to show on the order page as they are added not after the fulfillment is completed.
Thank you for your time.