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.
At the very least give an option to queue out of this "experiment". Dumb choice, if you didnt convince people earlier that YouTube was run by a bunch of goons up to now you are REAAAALLY trying to show how dumb you all are. Think smarter. Improve features people actually use next