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
First you demonetise two of my fav YouTubers, now you're making it so that their hard work in creating custom thumbnails is wasted on some of their viewers. This is a very, very STUPID idea and makes all of you who came up with it seem just as STUPID.