This happens to me too. As I can't reply to the newer support question, I'll do so here. It appears to occur only in visual editor mode. I can edit a document in HTML mode and put in XSLT samples e.g.
And save it fine. But if I accidentally switch to Visual mode the XSL elements will be stripped but the HTML elements remain. I'm guessing this has something to do with a filter that fixes invalid HTML. There is an option in Writing called "WordPress should correct invalidly nested XHTML automatically". But in my case that's currently disabled.