CDK - CodePipeline notification rule - Resource handler returned message: "Invalid request provided: AWS::CodeStarNotifications::NotificationRule"

We have a helper method in our CDK library (based on Amazon.CDK.Lib 2.28.1) to establish a CodePipeline notification rule to SNS topic that delivers events to a Microsoft Teams channel via Lambda function.

internal static void ToMicrosoftTeams(CodePipelineNotifierProps props)
{
	var topic = EstablishEventsTopic(props);
	props.Pipeline.NotifyOnExecutionStateChange("NotifyOnExecutionStateChange", topic);

	// Pull the lamdda function python code embedded in assembly.
	using var fileStream = Assembly.GetExecutingAssembly().GetManifestResourceStream("CdkLibrary.Lambda.CodePipeline_events_MicrosoftTeams.py");
	using var streamReader = new StreamReader(fileStream);
	var lambdaCode = streamReader.ReadToEnd();

	var funcName = $"{props.ProductName}-{props.PipelineName}-events-to-teams-{props.Environment}";
	var lambdaFunc = new Function(props.Pipeline, funcName,
		new FunctionProps
		{
			FunctionName = funcName,
			Runtime = Runtime.PYTHON_3_9,
			// Inserting code as raw string; resulting code file will default as index.py.
			// Function lambda_hanlder()
			Handler = "index.lambda_handler",
			Code = Code.FromInline(lambdaCode),
			Environment = new Dictionary<string, string>
			{
				{ "TeamsChannelUrl", props.TeamsChannelUrl}
			}
		});

	topic.AddSubscription(new LambdaSubscription(lambdaFunc));
}

private static Topic EstablishEventsTopic(CodePipelineNotifierProps props)
{
	string topicName = $"{props.ProductName}-{props.PipelineName}-events-{props.Environment}";
	return new Topic(props.Pipeline, topicName, new TopicProps
	{
		DisplayName = props.PipelineName + " events",
		TopicName = topicName
	});
}

This setup fine in our testing. However today in deploying a new environment, there seem to be a new error encountered on creating the notification rule.

AppService-stack | 3/10 | 2:59:02 pm | CREATE_FAILED | AWS::CodeStarNotifications::NotificationRule | AppService-pipeline-dev/AppService-pipeline-dev/NotifyOnExecutionStateChange (AppServicepipelinedevNotifyOnExecutionStateChangeE355A12F) Resource handler returned message: "Invalid request provided: AWS::CodeStarNotifications::NotificationRule" (RequestToken: <GUID>, HandlerErrorCode: InvalidRequest)

AppService-stack | 3/10 | 2:59:03 pm | CREATE_FAILED | AWS::IAM::Role | AppService-pipeline-dev/AppService-pipeline-dev/AppService-pipeline-events-to-teams-dev/ServiceRole (AppServicepipelinedevAppServicepipelineeventstoteamsdevServiceRoleDE6A960E) Resource creation cancelled

Why has this been considered an invalid request?