How to correctly update a map field in firestore using python?

I have a map in firestore and I want to regularly update it (not overwrite previous keys). Most of the times it works, however sometimes it fails and it does not throw any exception. The only indication that something went wrong is that the result (https://cloud.google.com/firestore/docs/reference/rest/v1/WriteResult) has an update_time which I can compare to now() and if the difference is too large I know it did not do an update now. The problem is after that the whole map is missing (all previous keys are gone). So not only it failed to add the current keys but somehow it wiped out the whole field. Below is the full code:

        error_keys = []
        for key in data.keys():

            # continue if set is empty
            if not data[key]:
                continue
            try:
                
                new_keys = {
                    f'myMap.{k}': v for k, v in data[key].items()}

                result = self.db.collection(u'myCollection').document(key).update(
                        new_keys
                )
               
                now = datetime.now(tz=pytz.utc)
                dt_string = now.strftime("%d/%m/%Y %H:%M:%S.%fZ")
                duration = now - result.update_time                         #
                duration_in_s = duration.total_seconds()
                minutes = divmod(duration_in_s, 60)[0] 
                if minutes > 1.0:  
                    logger.warning("Diff to update time is larger that 1 min")
                    logger.info(f'Now: {dt_string}')
                    logger.info(f'Duration in minutes: {minutes}')
                    logger.info(f'Adding {key} to error_keys')
                    error_keys.append(key)
                
                logger.info(f'KEY: {key}: {data[key]} Update_time: {result.update_time} Diff_minutes: {minutes}')

            except:
                logger.warning(
                    f'Could not key: {key} with data {data[key]} to firebase.')
                error_keys.append(key)
                logger.exception('Exception writing keys')

        logger.info(f'ERROR_KEYS: {error_keys}')
        return error_keys

I am using:
google-cloud-firestore 2.1.0 Python 3.7.3