How to publish plain text payload from AWS Lambda python function to IOT Core?

Hi. Are you sending 0 65432 0 192.168.2.1 502 5 1 6 338 0 as a string? I think that's what you mean. 37 bytes are sent, and 37 bytes received by subscribers. No characters added.

Lambda:

import boto3
import json

def lambda_handler(event, context):
    print('Hello')
    iot = boto3.client('iot')
    endpoint = iot.describe_endpoint(endpointType='iot:Data-ATS')['endpointAddress']
    print(f'{endpoint}')
    iot_data = boto3.client('iot-data', endpoint_url=f'https://{endpoint}')
    iot_data.publish(topic='repost', qos=0, payload='0 65432 0 192.168.2.1 502 5 1 6 338 0')

    return {
        'statusCode': 200,
        'body': json.dumps('Hello from Lambda!')
    }

And using mosquitto_sub as the subscriber because it reports the number of bytes:

@ubuntu:~/Desktop$ mosquitto_sub -h foobar-ats.iot.us-east-1.amazonaws.com -p 8883 -t repost --cert 8ffe5d98-certificate.pem.crt --key 8ffe5d98-private.pem.key --cafile AmazonRootCA1.pem -d
Client mosq-23ZjsfCHOnWUais1gn sending CONNECT
Client mosq-23ZjsfCHOnWUais1gn received CONNACK (0)
Client mosq-23ZjsfCHOnWUais1gn sending SUBSCRIBE (Mid: 1, Topic: repost, QoS: 0, Options: 0x00)
Client mosq-23ZjsfCHOnWUais1gn received SUBACK
Subscribed (mid: 1): 0
Client mosq-23ZjsfCHOnWUais1gn received PUBLISH (d0, q0, r0, m0, 'repost', ... (37 bytes))
0 65432 0 192.168.2.1 502 5 1 6 338 0

Perhaps I misunderstand your problem.