Oracle SOA 12c REST adapter content-type issue

In a BPEL process developed in SOA 12.2.1.2.0, we are consuming a REST api in the reference by using REST adapter WSDL approach. The resource operation is a PATCH call with a JSON payload in the request and empty response for success.

As per the API spec, we are required to pass "application/merge-patch+json" as the content-type in the HTTP request headers. We are doing this by setting the property (rest.binding.http.Content-Type) to value "application/merge-patch+json" in the invoke activity of the REST reference.

But while testing the service, we are receiving the below error while it is invoking the REST API which shows that an unsupported type fault is returned with a 415 http response. It shows application/json is getting sent in the HTTP request but even in the logs(REST adapter:TRACE 32) logger when verified we could see at run time that http header is getting passed as expected "application/merge-patch+json" but somehow in the actual request sent it is going with a"pplication/json" as evident in the 415 error.