YouTube introduced a new algorithm that scans videos for the best thumbnail image while it’s being uploaded, and then enhances their 3 thumbnail pics to be as optimized as possible. You can still upload a custom thumbnail, but the auto-generated thumbnails should be improving now based on quality scores YouTube assigns to each frame in the video. I share a bit of how the YouTube thumbnail algorithm picks the best images to suggest as thumbnail options.
New Wizardry that Creates Optimized Thumbnails Automatically
Improving YouTube video thumbnails