So I updated the BIOS on my PRIME X399-A from v403 to v407 today, but looks like Asus has screwed something up with it as it refuses to boot with memory set any higher then 2133MHz. Even with everything at stock and just the RAM set to 3200MHz using D.O.C.P. (which worked on the previous version) the system fails to post (shuts down and resets while the MEM boot-up light is illuminated) and after about 5 resets I get a POST error telling me that the system failed to boot due to memory settings which have been reset (to 2133MHz).
I tried re-downloading the BIOS and flashing again and have the same problem. I flashed back to v403 and everything works again.
Hopefully Asus will address this issue and release an updated BIOS for the PRIME X399-A soon.