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.
If we use a Custom Thumbnail and DON'T choose one of the 3 auto-generated thumbnails for our video when we upload it, why would we want our videos to automatically have auto-generated thumbnails instead, when we SPECIFICALLY decided against it? STOP BEING STUPID, YOUTUBE DEVS.