Accordingly JAX-RS specification, the parameters marked with @FormParam annotation can take the values from the query parameters in case if request body is already consumed:
Servlet Container section:
"Servlet filters may trigger consumption of a request body by accessing request parameters. In a servlet container the @FormParam annotation and the standard entity provider for application/x-www-form-urlencoded MUST obtain their values from the servlet request parameters if the request body has already been consumed. Servlet APIs do not differentiate between parameters in the URI and body of a request so URI-based query parameters may be included in the entity parameter."
It is reasonable default behavior, but for in some instances this may be undesired.
I would propose to introduce custom option to explicitly forbid specified assumption.