Some of this is that underlying model is insufficiently trained, but some of this is disambiguation. Disambiguation in text is a very, very, hard problem.

So some of your examples, when clarified, are a bit clearer: Paul McCartney - Beatles + Rolling Stones = Mick Jagger is in the 3rd spot. (http://www.thisplusthat.me/search/Paul%20McCartney%20-%20Bea...) Change stone -> Rolling Stones.

Thank you for the comment!