Stop bullying me @TeamYouTube & give me my thumbnails back!pic.twitter.com/4em3d1kpLE
You can add location information to your Tweets, such as your city or precise location, from the web and via third-party applications. You always have the option to delete your Tweet location history. Learn more
Then maybe they won't have stats to compare and based things on. As in, if a video doesn't have custom thumbnail then how can you know it will perform worse or better with their experiment. While the ones with custom, they will be able to compare the before and after.
They are testing auto-generated thumbnails nothing to do with custom thumbnails or anything to do with performing against custom thumbnails. Clickbait thumbnails will always perform better against auto generated thumbnails.
Twitter may be over capacity or experiencing a momentary hiccup. Try again or visit Twitter Status for more information.