Reading through the comments of other people having similar problems makes me question if this is at all related to the specific issue. But I think it's worth bringing up anyway:

When I first started using PiHole, the default filters were filtering out all of the YouTube tracking domains. Which makes sense. However, these are the same domains that the history tracking uses. So it would never remember that I saw a video or how far into it I got.

Perhaps the Not Interested gets similarly blocked by tracking filters?