Here is what the 286 spec has to say on the matter of renderParams (portlet 2.0 final, p. 77):

Note that render parameters get automatically cleared if the portlet receives a
processAction or processEvent call and need to be explicitly re-set on the response of
such a lifecycle call.

The clearing of renderParams is a required behavior the 2.0 specs; it is explicitly and positively stated.   If you're going to depart from the spec, you should:
1.  warn people, and
2.  provide some kind of switch to revert to spec-compliant mode.

Some further adivce:
1.  Don't offer a partial implementation of specs;  it is confusing -- especially when that partial implementation alters established behavior.
2.  Have better test coverage.   The 2.0 spec makes 7 statements about RenderParams, and your tests had (apparently) no coverage for one of those statments.   This is an obvious and non-trivial part of how renderParams behave, and anyone who has actually used them in real life will expect it.
3.  Please remember that some organizations have large investments riding on spec conformance.   Inadvertant divergence from spec conformance does not inspire confidence.  


If anyone wants their Liferay 7.x portlets to conform to the 2.0 spec (at least in regards to render parameters), extend Porlet2ConformantPortlet (below).   It's a somewhat hacky fix, and I haven't had a chance to do much testing (use at your own peril), but it does appear to work.  

import com.liferay.portlet.PortletRequestImpl;

public class Porlet2ConformantPortlet extends GenericPortlet {

    @Override
    public void processEvent(EventRequest request, EventResponse response) throws PortletException, IOException {
        ((PortletRequestImpl) request).clearRenderParameters();
        super.processEvent(request, response);
    }

    @Override
    public void processAction(ActionRequest request, ActionResponse response) throws PortletException, IOException {
        ((PortletRequestImpl) request).clearRenderParameters();
        super.processAction(request, response);
    }
}


Your extending portlet will, of course, have to call the super methods for processAction() and processEvent(), and make this call immediately, before doing anything else.

Thanks,
Ross