As of January 30, 2019, requests to the Get Order and List Orders APIs may return orders with the null status. That result wasn’t previously available while using the referred APIs.
Main advantages
The new status allows order closing, even when the Workflow is facing stability issues. Before that change, the Workflow (our order status management system) would impact the checkout system, preventing orders to be closed properly. Now, orders are closed as they're supposed to, and get the null status until the incident is resolved.
In other words, the new status is responsible for the Workflow's optimization and makes the order closing process more stable.
What you need to do
You must review all your store’s integrations that use the Get Order and List Orders (like ERP integrations, for example). The ones that, for any reason, are not ready to receive the new response must be adjusted before January 30.