This is pretty odd!
Can you do me a favour - I realise that you have OpenGraph markup in your page already, so this shouldn’t be necessary, but would you mind trying adding twitter:title and twitter:description tags into the header as well, and trying the validator again? I’m very curious about what might be happening.
The other thing to note is that I don’t think you have a robots.txt file at the moment, which might impede the Twitterbot from getting to all of the content.