Hi,
I was having exactly the same issue. It turns out the syntax isn't the same as on the Liquid docs. You need to use:
"executionDate": "{{ content.ExecutionDate | Date: "dd/MM/yyyy" }}", notice the capitalisation of "Date"
See following article for details:
https://social.technet.microsoft.com/wiki/contents/articles/51275.microsoft-azure-liquid-templates-in-logic-apps.aspx
I hope that helps!