Im creating a POC with nested Step Functions (SF) and have hit the following issue.
I ran into the same issue. I used "OutputPath": "$.Output" at first and that returned the escaped json. I removed that and added "ResultPath": "$.Output", This works, it returns the output as json including the escaped version. Its ok for now until AWS addresses this.
Not sure it's new since you asked the question but the answer is in the link you provided: Use "arn:aws:states:::states:startExecution.sync:2" in the task resource field instead of "arn:aws:states:::states:startExecution.sync"