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
That's odd! Please send a feedback report directly from YouTube with these steps: https://goo.gl/WdI0BK . While every report may not get a direct response, we review everything and use the details to investigate and fix issues.