This week's book giveaway is in the Reactive Progamming forum. We're giving away four copies of Reactive Streams in Java: Concurrency with RxJava, Reactor, and Akka Streams and have Adam Davis on-line! See this thread for details.
JSON.stringify() is not a jQuery API, so if there were an error in generating the JSON (unlikely), the problem would not be with jQuery.
If this were my problem I'd be using the browser tools to see the request that is being sent to make sure that it contains what is expected. If it does not, then the problem would be somewhere on the front end while generating the request. If the request is as expected, the problem is likely on the back-end, perhaps with the REST mappings.