For the next few weeks, 0.3% of your viewers on YouTube won’t see your custom thumbnails.https://socialblade.com/blog/youtube-auto-generated-thumbnails-experiment/ …
I think @TeamYouTube should leave that kind of tinkering to the creators. If a video I upload performs terribly I might try changing the thumbnail myself. Let me be in control, stop with this constant manipulating! There's no benefit to anyone, yet @youtube keeps at it.