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.
Cause you know, as a small channel trying to get off the ground, having spent time and money creating thumbnails, I really love hearing stuff like this is being done without first asking... anyone who uses your platform.