Currently, we assume that the server's command response to a changeStream aggregation query will contain the operationTime field. Consequently, we record the start_at_operation_time parameter upon change stream creation as follows:

self._start_at_operation_time = result["operationTime"]

However, it seems that the server might not always include the operationTime field in its command-responses (see e.g. PYTHON-2177). Consequently, it might be a good idea to not rely upon the presence of this field:

self._start_at_operation_time = result.get("operationTime")

We should consider making this change and also updating the change streams specification once we get a definitive answer to HELP-14870