Thumbnails are a strange kind of photo
A TikTok cover frame is usually mid-motion, mid-expression, often mid-sentence, and frequently has huge text plastered across it. That's harder to match than a posed portrait, so TikTok results tend to score lower than Instagram or LinkedIn results for the same person. Treat a 78 on TikTok as more meaningful than a 78 on a clean headshot — the platform is fighting you.