I'm pretty late to this, but the navi firmware didn't work on previous firmware versions since it was too big to fit on bootkernfw. I swapped my rx5300 out for an rx560 and loaded the polaris fw to bootkernfw, and it worked perfectly fine for me- I get a picture in petitboot, and I boot to desktop in fedora without any issues. Not sure if it's something with navi cards, or just mine in particular (I don't have any other ones I could test it with). Guess I'll just stick with the rx560 for now.