Lambda CloudWatch log group not created when used existing role of other lambda

Hello.

It is highly likely that the IAM policy set for the IAM role "lambdaA-role-123" is configured to create "/aws/lambda/lambdaA".
By default, Lambda uses a log group named "/aws/lambda/<function name>".
https://docs.aws.amazon.com/lambda/latest/dg/monitoring-cloudwatchlogs-loggroups.html

By default, CloudWatch automatically creates a log group named /aws/lambda/<function name> for your function when it's first invoked. To configure your function to send logs to an existing log group, or to create a new log group for your function, you can use the Lambda console or the AWS CLI. You can also configure custom log groups using the CreateFunction and UpdateFunctionConfiguration Lambda API commands and the AWS Serverless Application Model (AWS SAM) AWS::Serverless::Function resource.

When you create a Lambda function from the management console with default settings, the IAM policy will also be set accordingly. As a result, if you reuse the IAM role in other Lambda functions, you may encounter a problem where logs cannot be output.

In other words, it appears that the IAM role "lambdaA-role-123" has the following IAM policy configured, which restricts logs to being output only to "/aws/lambda/lambdaA".

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "logs:CreateLogGroup",
            "Resource": "arn:aws:logs:your-region:AWS-Account-ID:*"
        },
        {
            "Effect": "Allow",
            "Action": [
                "logs:CreateLogStream",
                "logs:PutLogEvents"
            ],
            "Resource": [
                "arn:aws:logs:your-region:AWS-Account-ID:log-group:/aws/lambda/lambdaA:*"
            ]
        }
    ]
}

To resolve the issue, you need to either increase the number of log groups to output to using IAM policies, separate IAM roles and configure IAM policies to output to "/aws/lambda/lambdaB", or change the log group to output to in the Lambda settings.

and 2) logs of lambdaB were getting created in the CloudWatch log group /aws/lambda/lambdaA

By the way, regarding issue #2 that you encountered, I was unable to reproduce the same problem when I tried it on my AWS account.
Regarding issue #2, I believe this issue will not occur unless the log output destination has been changed in the Lambda function in question, and the IAM role used by the Lambda function has permission to access the target CloudWatch Logs log group.
To reproduce the problem, I think I would need to know the contents of your Lambda's IAM settings and log output settings.

By design, AWS Lambda automatically creates a separate CloudWatch log group /aws/lambda/<function-name> for each function when it is first invoked, provided the execution role has the correct CloudWatch Logs permissions. If you reused the same execution role across multiple functions without properly scoping the IAM policy, logs may be written to the wrong group or fail to create. This is expected behavior if the IAM policy only grants access to one log group.

https://docs.aws.amazon.com/lambda/latest/dg/monitoring-cloudwatchlogs.html

https://repost.aws/knowledge-center/lambda-cloudwatch-log-streams-error

https://stackoverflow.com/questions/67842484/log-group-does-not-exist-error-when-using-aws-lambda