I've had this problem happen to me as well but I'm not sure exactly what causes it. It's inconsistent, but I can sometimes get it to happen by turning a large group of func_details back into world brushes all at once. If you've done that recently, that might be it.
You might want to make a copy of it and see if it's still happening in the copy. If not, just delete the original and use the copy. If it is broken in the copy, delete one brush at a time from it and test it each time. Once you find the problem brush when the visual glitch goes away, delete that corresponding brush from the original, and remake that one from scratch. It's a hassle, but I don't see why that wouldn't fix it.