Does not seem to be fully working. Maybe this is a Youtube issue. Youtube usually shows a red line showing how much of a video you have seen. That red line seems to be missing on videos I have watched a long time ago.
One example just now was a video in my Home - Dave Brubeck plays Golden Brown. Hmm, I think, just 2:19 and I like both Brubeck and the song. Then I see that I have already liked the video, so I have clearly watched it before, though probably a long time ago and there was no red line showing although I would have watched it all last time I saw it.