S3 trigger with prefix in lambda is not working

According to https://docs.aws.amazon.com/AmazonS3/latest/userguide/notification-how-to-filtering.html, the suffix must NOT include the leading "." (dot). So suffix should be json

However, I tested with both (including the leading . and without), and it both worked.

I used the following AWS SAM template:

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: >
  sam-app

Globals:
  Function:
    Timeout: 3
    MemorySize: 128

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function 
    Properties:
      CodeUri: hello_world/
      Handler: app.lambda_handler
      Runtime: python3.9
      Architectures:
        - x86_64
      Events:
        BackupEvent:
          Type: S3
          Properties:
            Bucket:
              Ref: MyBucket
            Events: s3:ObjectCreated:*
            Filter:
              S3Key:
                Rules:
                - Name: prefix   
                  Value: presets/
                - Name: suffix   
                  Value: .json  
      Policies:
        - S3ReadPolicy:
            BucketName: MyBucket

  MyBucket:
    Type: AWS::S3::Bucket
    Properties:
      BucketName: examples3eventbucket
    

with the following Lambda function in python:

import json
import urllib.parse


def lambda_handler(event, context):
   print("Start processing S3 Event")
    
   file_name = urllib.parse.unquote_plus(urllib.parse.unquote(event['Records'][0]['s3']['object']['key']))  #S3 event contains file name in URL encoding, needs to be decoded - https://github.com/aws-samples/amazon-textract-enhancer/issues/2
   print("Backup file: " + file_name)

and I uploaded a file using aws s3 cp abc.json s3://examples3eventbucket/presets/xyz/abc.json

and check the Lambda logs using sam logs --stack-name sam-app , you can see the Lambda function was invoked:

2023/10/12/[$LATEST]66b884464faf4c3e98af033f4e8b6272 2023-10-12T08:46:26.569000 INIT_START Runtime Version: python:3.9.v32      Runtime Version ARN: arn:aws:lambda:af-south-1::runtime:65d8a55e7d094cdfca79c506263a99c166ff4afba537e80b4eec2eb9568f7ba6
2023/10/12/[$LATEST]66b884464faf4c3e98af033f4e8b6272 2023-10-12T08:46:26.674000 START RequestId: fff97e6f-d7e9-46dd-93e7-3b07635f6dca Version: $LATEST
2023/10/12/[$LATEST]66b884464faf4c3e98af033f4e8b6272 2023-10-12T08:46:26.674000 Start processing S3 Event
2023/10/12/[$LATEST]66b884464faf4c3e98af033f4e8b6272 2023-10-12T08:46:26.674000 Backup file: presets/xyz/abc.json
2023/10/12/[$LATEST]66b884464faf4c3e98af033f4e8b6272 2023-10-12T08:46:26.675000 END RequestId: fff97e6f-d7e9-46dd-93e7-3b07635f6dca
2023/10/12/[$LATEST]66b884464faf4c3e98af033f4e8b6272 2023-10-12T08:46:26.675000 REPORT RequestId: fff97e6f-d7e9-46dd-93e7-3b07635f6dca  Duration: 1.55 ms       Billed Duration: 2 ms   Memory Size: 128 MB     Max Memory Used: 36 MB  Init Duration: 103.19 ms