TMS320F280049C: Reading the SCI RX buffer RXFFST number of times doesn't decrement RXFFST to 0

I must not understand how the SCI FIFO works because when I try to empty the Rx FIFO by reading out the number of words indicated by RXFFST, RXFFST still equals 1.  I think it should be zero and I think this leads to an OVF condition eventually.

uint16_t fifo_len = sci_reg_->SCIFFRX.bit.RXFFST;

for (i = 0; i < fifo_len; i++) {
rx_data_[rx_ready_index_ ^ 1][rx_index_ + i] = sci_reg_->SCIRXBUF.bit.SAR;
}
// RXFFST isn't 0