BUG: no access to S3 Express One Zone (aka directory buckets) from AWS Lambda

Accessing buckets in S3 Express One Zone (aka directory buckets) from AWS Lamba function is not working. My Lambda function is based on the example from the documentation:

import boto3
import json

def get_object(s3_client, bucket_name, key_name):
    try:
        response = s3_client.get_object(Bucket=bucket_name, Key=key_name)
        body = response['Body'].read()
        print(f"Got object '{key_name}' from bucket '{bucket_name}'.")
        return body
    except Exception as e:
        print(str(e))
        raise 

def lambda_handler(event, context):
    s3_client = boto3.client('s3')
    resp = get_object(s3_client, 'mybucket--use1-az4--x-s3', 'myobject')
    return {'statusCode': 200}

I get an exception at calling get_object:

An error occurred (NoSuchBucket) when calling the GetObject operation: The specified bucket does not exist.

If I use the name of a regular (non-directory) bucket, it works.

The issue has been corroborated at SO.