Genre tag "Pop" is changed to "[Pop"

I had this issue with iTunes 10.4.x, then it was absent in 10.5, but now it's back in 10.5.1. I have lots of tracks using "Pop" genre tag, but when I edit or listen those tracks, the genre is automatically changed to "[Pop". I can see the correct "Pop" tag in genre dropdown menu, but when I choose that it shows as a "[Pop" in genre field. If I put something completely different to genre field, everything works correctly, so for some reason the genre value "Pop" is damaged. What I could try to do for fix?