Why cc1200 rx fifo can not been cleaned occasionally?

I wonder if anyone else has ever met with the same case, the command stobeSFRXcould not clean the rx fifo of cc1200 successfully , sometimes and somewhere. "sometimes" means not always, and "somewhere" means, I executed the SFRX command at 6 different places of my source code project and just found this error occur at two of them. Let me show more details.

I usedinfinite packet lengthto send and receive data. The rx fifo is cleaned when unexpected abort during receiving, when packet receiving finished, when cc1200 initialised, when unexpected abort during transmitting… The error occurred just when unexpected abort during receiving. "unexpected abort" might be cases such as some unexpected data was received or rx/tx fifo error was detected.

In the whole source code, I clean the rx fifo with three sentences listed below: set cc1200 to idle state, send command SFRX, check if cc1200 is in idle state.

CC1200P__cmd_strobe(CMD_SIDLE);
CC1200P__cmd_strobe(CMD_SFRX);
while ((CC1200P__reg_read_robust(REG_MARCSTATE) & 0x1f) !=0x01) ;

I found the rx fifo not cleaned as the value of cc1200 register NUM_RXBYTES ,read out exactly after the three sentences above, does not equal to zero. Sometimes I got 0x0B, 0x0C or other values.