I just spent about an hour trying to figure out why the heck the card validator was failing. It seems that twitterbot has some buffer with a max size of 32768 bytes which contains the page plus some other data, and if you give it more data than that, it fails to parse the meta tags.
This is a bug. Where can I report it?
If you go to https://cards-dev.twitter.com/validator and enter them in, the second one returns
INFO: Page fetched successfully
WARN: No metatags found
the only difference between the pages is a few extra characters, and it doesn’t seem to be content dependent.