Saving to Cloud Firestore on iOS hangs when offline

Loading data from firestore while offline works as expected but a call to save never returns and there seems to be no timeout either.

This is a example save that works online but not offline:

func  save() {
    guard let uid = user?.uid else {
        return
    }
    let db = Firestore.firestore()
    var ref: DocumentReference? = nil
    ref = db.collection("users").document(uid).collection("properties").addDocument(data: ["name": "test"]) { err in
        if let err = err {
            print("Error adding document: \(err)")
        } else {
            print("Document added with ID: \(ref!.documentID)")
        }
    }
}

Is there any known workaround?

UPDATE: Firebase support have confirmed it's a bug and that it "is now being worked on by our engineers". They are unable to give a timescale for when it will be fixed.