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
Stop messing with the system, instead of making new problems fix the ones you already got @TeamYouTube (last time i checked you still havent fixed sub boxes or false copyright strikes)