With actual rules http://www.subX.dom.com is redirected to http://subX.dom.com,

With the rule I proposed? Somehow I have a very hard time believing that, since my rule never shortens an URL.

Are you sure you actually tested it? And that you didn't use a browser to test it, but a tool like wget?

(Browsers cache permanent redirects, so they are not very useful for debugging them).