Cognito Migration Trigger errors when Lambda execution time too high

I am currently in the process of validating the migration of a set of users to a cognito user pool via the migration trigger, the essence of the lambda function for the trigger can be boiled down to:

def lambda_handler(event, context):
   response = requests.post(external_auth_api_url, json_with_user_and_pass)
   if response.status_code = 200:
      event["response"] = {
            "userAttributes": {
                "username": event["userName"],
                "email": event["userName"],
                "email_verified": "true"
            },
            "finalUserStatus": "CONFIRMED",
            "messageAction": "SUPPRESS"
        }
      return event

This is doing an external rest call to the old system the user was signing in through as per the documentation and returning a success response.

The issue I noticed is that if the lambda function time is too long, for example, the average execution time of this lambda for me right now via ngrok is about 5 seconds total, cognito is failing when I call initiateAuth with USERNAME_PASSWORD flow and returning the following:

botocore.errorfactory.UserNotFoundException: An error occurred (UserNotFoundException) when calling the InitiateAuth operation: Exception migrating user in app client xxxxxxxxxxxx

I managed to validate that this issue was occurring by simply returning a success response without doing an external REST call and essentially bringing the lambda function runtime down to milliseconds, in which case I got the tokens as expected and the user was successfully migrated.

I also tested this by simply having a lambda function like:

def lambda_handler(event, context):
    time.sleep(5)
    event["response"] = {
            "userAttributes": {
                "username": event["userName"],
                "email": event["userName"],
                "email_verified": "true"
            },
            "finalUserStatus": "CONFIRMED",
            "messageAction": "SUPPRESS"
        }

    return event 

This fails with the same error response as above.

If anyone can advise, I am not sure if there is a maximum time the migration trigger will wait that is not documented, I wouldn't expected the trigger to have such a thing if the migration trigger's intention is to do external REST calls which may or may not be slow.

Thanks in advance!