We are running a small experiment where 0.3% of viewers will see an auto-generated thumbnail, instead of your custom thumbnail. We are not removing the ability to create your custom thumbnail, but we hope to gain insights on auto-generated thumbnails for the future.
This is a complete waste of time and resources. As a consumer I hereby demand you cease these useless experiments that hurt the creators I follow. @Vimeo is looking like a better platform everyday.