Way back when I used Pandora I found similar behavior if I gave songs thumbs down. It would converge on a small selection of a few artists and their music. It was really good at some genres if I didn't thumbs up or down many songs.

I've seen the same kind of thing in other services that weight recent choices as a part of their graph based recommendations like Spotify and YouTube. They can get stuck in a feedback loop where the pool of recommendations shrinks to the point where it recommends the same stuff over and over.