Hi, Xia,
I am sorry for the delay.
Regarding your issue, pls refer to the RXEDGIF bit description, the RXEDGIF bit is set once it detects active edge on RxD pin, it is cleared by writing a 1 to the bit. But you write a 0 to the bit, that is why you can not clear it.
Hope it can help you
BR
Xiangjun Rong