We heard this a lot from @TeamYouTube during their "Mythbusting" panel - It was just an experiment, bro.
I get you want to test ways to improve the site, but we're no lab rats, we're partners. Please stop harming our BRAND against our will; ask for volunteers.https://twitter.com/TeamYouTube/status/1012160856690450432 …
It's almost like they want to change the thumbnail on videos they don't like in the future.