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.
Don't do this, it's pointless and unneeded. Just host the videos, lets us skip adds over 30 seconds and stop pushing a premium account everytime I switch apps, if I wanted it I'd get it.