If a client sends an accept header like application/json but the server answers with content type application/json;charset=UTF-8 (Spring does this by default), then the client cache won't work.

I would say that the problem is, that the content type of the response is used when adding the first response to the cache. But this might not be what the client asked for. Even if the content type of the response is text/xml when asking for application/json, it should still be cached under application/json.