Ty for your answer, but actually I need to use exactly the JSON in this format (despite it’s not correct). This was the reason that I told it worked on SoapUI. My legacy REST service needs a “custom JSON” like this to work out.
I’ve found a workaround to avoid this kind of verification. I’ve put all my JSON file into a string, and used that in the HTTP Response body. Doing that all “standard” format verifications was bypassed.