When an external object in Mendix is created then “saved/committed” via the “Send External Object” action, the attributes of the object should update with what comes back from the response.
OData’s standard has its POST endpoint for creating objects return the full object back upon successful creation. The reason this is, is because most systems have keys and identifiers generated by their database or by an external service. If an external object is sent I would expect the object in memory in my flow to be updated with the key that comes back upon creation.