MSP430FR6989's UCRXIFG0 question in IIC function

In the (eUSCI) – I2C Mode, as the userguide show "The UCRXIFG0 interrupt flag is set when a character is received and loaded into UCBxRXBUF","UCRXIFGx is automatically reset when UCxRXBUF is read".

So I want to check the UCRXIFG0 flag to read the  UCBRXBUF data in the main loop.code is following:

while(1)

{

while(!(UCB0IFG & UCRXIFG0));

RxData = UCB0RXBUF ;

}

But I found it is not successful !!!  

So I check the  UCB0IFG register in the USCI_B0_ISR(void) . I found UCRXIFG0 is set when the UCB0RXBUF is read and UCRXIFG0 is not set when the program into the receive Interrupt service function!!! What's wrong???