For example: https://gmb.io/ff/
The Twitter meta on the page is correct. It is valid according to the https://cards-dev.twitter.com/validator
curl -A Twitterbot https://gmb.io/ff/ returns the correct markup.
The image is big enough.
And both the robots.txt on the site and the CDN allow for the Twitterbot and is in proper plain text with ASCII encoding:
And I’ve gone over everything else here: Not Whitelisted, unable to render, or no image: READ THIS FIRST to no avail.