I'm of two minds on this.
I. On the one hand, she could be down-playing her accomplishment, trying to say "it's not that big a deal." Reasons of this could be things like (a) it's not a "cool" activity so she doesn't want to admit to being thrilled by it; (b) it means a LOT to her. So much that it's personal and private and she doesn't want to be pushed on it or congratulated or anything that would require talking to somebody else about the accomplishment. FYI - this is my son. We learned very, very early that if we do typical parental congrats, he'll just say "that's a Mom-love statement," and he'll even drop it if we talk about it too much; (c) well - she's 12. Nearly anything could be going on in the head.
II. On the other hand, this could be a sort of accolade-grab that she actually does NOT care about for itself. Certainly there are many kids who do this. Usually this starts because the kids are pushed by parents into a laudatory activity they don't really have any passion for. But they're good kids and capable so they excel and win. Then that cycle repeats and accelerates with the sort of icky potential that HK discussed. And it's frankly selfish - though I don't think the kids usually realize they are being selfish. It's just a path of least resistance they fall onto.
So - sorting this out probably requires a chat with her. I'd want her to understand that if she is in the II space, she's probably giving into social pressure in a way that hurts herself and others. If she's in the I space, counsel her not to downplay her accomplishments. Gracefully accepting accolades is a skill, and she can follow-up with "but goodness I'd really rather talk about something else. Are you done with your Christmas shopping [or some such re-direct]."
In any event to her - congrats to her! I hope she has fun playing music with other accomplished musicians. It's a soaring feeling!
Sue
