I'm pretty certain this is not a MediaWiki bug, but one induced by the namespace settings on English Wikipedia. (I couldn't reproduce it on my local wiki, for example).
This image shows what I see in "Edit watchlist" page:
As you see, there is a deleted page in Wikiepdia namespace, which is incorrectly listed under (main) namespace. This only happens to this one page. If I add new pages from Wikipedia or WP namespace, they get listed correctly. All pages from Wikipedia namespace which weren't deleted were also listed under the correct namespace (before I purged my watchlist to take this snapshot). I'm afraid this only happens to those pages in WP namespace which are now deleted. I also belive this has something to do with the recent changes in English Wikipedia namespace system, which made WP and alias of Wikipedia namesapce.
If there is some more information I can provide, I'd be glad to.
Version: 1.12.x
Severity: normal