The documentation of $setWindowFields should specify whether there are any guarantees for the output order of documents. Intuitively, you would assume that the documents are ordered by { partitionBy : 1, sortBy : 1 }, and in practice, this seems to be the case. However, this is not explicitly mentioned in the documentation, so developers cannot rely on it.

Edit: Here is the link to the page: https://www.mongodb.com/docs/manual/reference/operator/aggregation/setWindowFields/