The X-Spam-known-sender header means they're a sender you know (are in your address book) not that they're a known sender of spam
but that shouldn't have happened, it seems there's a bug occurring right now.
It looks like every email is getting an "X-Spam-known-sender: yes" header, as of around 7 hours ago (1/12 9:20am UTC is the first one I received with it, 8:57am UTC is the last one without it, so it started happening somewhere in that 23 minute window), and every message with that header gets past the spam filter.
Can't help you with the earlier ones with lower scores though.