Clicking on the tag "open thread" on this post only shows open threads from 2011 and earlier, at "http://lesswrong.com/tag/open_thread/". If I manually enter "http://lesswrong.com/r/discussion/tag/open_thread/", then I get the missing open threads. The problem appears to be that "http://lesswrong.com/tag/whatever/" only shows things posted to Main. "http://lesswrong.com/r/all/tag/open_thread/" seems to behave the same as "http://lesswrong.com/tag/open_thread/", i.e. it only shows things posted to Main, despite the "/r/all". Going to "article navigation → by tag" also goes to an open thread from 2011, so it seems to also ignore things posted to Discussion.