Some years ago I had a similar problem with PrettyFaces and MyFaces Orchestra. The problem back then was that Orchestra always appended a query parameter and was ignoring the already existing one. Not sure if this is a similar problem.
I suggest to debug RewriteViewHandler.getActionURL(). I’m not sure if Weld uses ViewHandler.getActionURL() or HttpServletResponse.encodeURL() to add the parameter. Unfortunately the interaction between all theses components is very difficult to debug. 🙁