I've seen video thumbnails with that a red bar baked-in to look like I'm halfway done watching them.

Brilliant hack. My eye naturally glances at those videos because it usually means I was simply distracted and probably want to resume it.

Except when it doesn't and you click on it again later only to realize "wait, I watched this."

I'm really confused as to why they do this: there must be hundreds of people and hundreds of millions of $s working on the YouTube algo. I'm seemingly in the minority that thinks it's largely brilliant, it regularly uncovers things that are related to stuff that I'm interested in but new takes on them, and sometimes throws up some absolute genius recommendation from nowhere. However, like you, my "Up Next" selection is plagued by videos I've previously watched! It's horrible UX.

I have two behaviours; TV-program-style videos I'll watch once, and music-style videos I'll watch repeatedly.

My theory is they have difficulty telling the difference, so they think the fact I've watched "80s-90s Hip Hop & R&B Playlist" repeatedly indicates I'd also like to watch "Plumbing bottle air vent teardown" repeatedly.