I just tried uploading your page to a different server and it worked fine in the validator. My working assumption would be that something about the way your server is responding to the cards crawler is causing the meta tags to be stripped out. I don’t understand what that could be, since when I fetch the page using curl -A Twitterbot they show up fine.
Unfortunately a lot of folks are out on vacation at the moment so I’m not sure what more I can do to help debug this today.