Thanks @NobbZ ,
It was happening because of the sigil , I found it finally and solved it by replacing the body of the inner json into a String that would contain a place holder for it.
So my requirement here was the value of a key in the map would be another json , that json would also be derived from a map , I used Jason.encode to do this , it puts an escape \" before the starting braces ,
and then after composing the body of the final json I used sigil (~s) which had put another escape prefixing the escape , which was causing a problem during Jason.decode since it was unable to find the start of the inner map.
How I fixed it is in the final json I just put a placeholder <body> and then replaced the body with the Jason.encode inner json string.
With sigils is there a way to avoid escaping the escape characters to construct the right json that can be used flawlessly with Jason.decode
