If you create a CloudWatch Rule/Event through CloudFormation and assign it to a Lambda function, it will get into a slightly broken state where the Lambda function doesn't think it has a CloudWatch Rule/Event attached to it (and won't execute on that schedule) but the Rule/Event will think that it is associated with the Lambda function and won't allow you to add the Rule/Event again.
- Use this reduced test case CloudFormation stack to recreate the bug
- Then go into the CloudWatch Rules/Events section and find the new Rule/Event that you just provisioned (probably called testcase_every-hour)