I encounter the problem the first word of the transfer is uncertain this afternoon. finally I find out where the bug is, here I share it to you all.
the problem may appear when you configure the SLWR active high.
we know the default configuration of FIFOPINPOLAR is active low. so it is active low before you change it. but you may have the mistake that the SLWR is always active high. so you initiate and generate the SLWR signal as it active high all the time. but this would write one word into the FIFO when you configure the SLWR active high. so the first word always seems uncertain
Thanks for sharing your debugging experience here.