According to the CDK reference documentation the Canary schedule expression only supports rate.
https://docs.aws.amazon.com/cdk/api/latest/docs/@aws-cdk_aws-synthetics.Schedule.html

According to this documentation the scheduling configuration is much more flexible https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Synthetics_Canaries_cron.html

This is a 📕 documentation issue