I have experienced this for months. I'd search for something obscure and it would show a few results with the rest being videos I had watched partially or things I had liked before. I guess the idea was to just return a bunch of videos with the idea being maybe I'd be willing to watch something else or to meet a bare minimum of results to display.
I was searching for comparison of kosher salt brands on youtube and noticed they are injecting suggested videos from my homepage feed into the search results. I can't wrap my head around how incredibly shady this is. I'd really like to slap whoever approved this 'feature'.