The voting system for full video labels does not apply on thumbnails, and is leading to videos (mainly ones that are unpaid/self-promo) having a different label on the thumbnail when browsing YouTube than the label that appears on the video watch page by the title or in the SB pop-up of the currently playing video.

Unless a full video segment is considered hidden by SB when it gets 2 down-votes, and the full video label takes categorical precedence, (i.e. sponsor > self/promo) then it will be selected to show on that video's thumbnail, even if the other full video segment has a higher up-vote count. Also, only the full vid label with the most votes appears in the SB pop-up, so most users can't down-vote an incorrect full vid segment unless using a tool like SB Control and getting the segment id from SB Browser.

There's a cache for thumbnail labels, so changes take a while to update compared to the extension pop-up, but the video below had its full video label for Unpaid / Self-Promo submitted over three hours ago and other videos with newer FV segments had their labels update since then.
Vid below: (Unpaid/Self-Promo Vote: 0 | Sponsor vote: -1)

How the same video looks elsewhere: