Looks like you're in a small and brief experiment (.3% of viewers for a few weeks) so you're seeing auto-generated thumbnails instead of custom thumbs. This is part of an effort to test/improve auto-generated thumbnails for the many creators that use them: https://goo.gl/A7oDmr
I did not consent to all youtube video thumbnails looking like garbage to me and not having the option to even turn it off. This is incredibly disrespectful towards content creators who do a lot of work to have nice thumbnails.