Hi Jun,
I have been dealing with the same issue - it was also present in iOS 5, but I have been able to reproduce it 2540 to 2540, so I do not think it is iOS specific. I am using an ugly work-around for now. T.I. employee Tim asked me for more details in the thread below, but I have not had time to revisit the issue. Perhaps you could provide Tim with the requested info so he can look into the issue.
see this thread -> http://e2e.ti.com/support/low_power_rf/f/538/p/183876/672158.aspx#672158
-Tyler