Missing document properties!

    while ( archiverCount > 0 ) {
        messages
         .find( )
         .where('state').equals('REMOTE')
         .where('is_spam').equals(false)
         .where('spam_score').lte(MAX_SPAM_SCORE)
         .where('deliveryAttempts').lte(MAX_DELIVERY_ATTEMPTS)
         .where('nextDeliverAttempt').equals(null)
         .sort( ARCHIVE_SORT )
         .exec( )
         .then( async (msgDoc) => {
            if ( msgDoc === null || msgDoc === undefined ) {
                log.error('msgDoc provided in .then() is NULL!');
                console.error('[' + getTimestamp() + '] msgDoc provided in .then() is NULL!');
                // By calling return this early in the async function, we're essentially calling
                //  'continue' to iterate through the while() loop again.
                return false;
            }

console.debug('msgDoc (inside while() loop): ' + JSON.stringify(msgDoc));
            logEntry = '[' + getTimestamp() + '] Beginning message archival process (id: ' + msgDoc._id + ').';
//
// W-T-F ??
// The call to console.debug() that references 'msgDoc' /sees/ the document and all of
//  it's properties; but then by the /very next/ line, accessing one of those properties
//  (the '_id' property) returns undefined.
//
console.debug(logEntry);
console.debug('msgDoc: ' + JSON.stringify(msgDoc));
console.debug('msgDoc._id: ' + msgDoc._id);
            res = await messages
             .updateOne({'_id': msgDoc._id},
             {
                '$push': { 'Log': logEntry },
                '$currentDate': { 'lastUpdated': { '$type': 'date' }}
             })
             .exec();
            if ( res.modifiedCount != 1 ) {
                // modifiedCount should NEVER be greater than 1 (since we called findOne()
                //  nor should it ever be zero; in either case, something is awry.
                console.log('[' + getTimestamp() + '] messages.updateOne() modified count != 1: ' +
                    res.modifiedCount + '.');
                log.info('[' + getTimestamp() + '] messages.updateOne() modified count != 1: ' +
                    res.modifiedCount + '.');

                dbLog.create({
                    'application': APP_NAME_FULL,
                    'ts': Date.now(),
                    'host': fqdn,
                    'pid': process.pid,
                    'message': 'messages.updateOne() modified count != 1 (value: ' + res.modifiedCount + ')'
                });
            }