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.
I watch people like @NorthernlionLP who has hundreds of episodes in a series so having a random screen grab from the video is utterly pointless. It's much easier to just see at a glance the number in the thumbnail.