I had to join to post my experience. Lurked here for years. This is just too important.
I tried BIOS 0503 direct from the Asus X299-A motherboard support page; coming from 0402.
BIOS 0503 caused a boot failure in my system.
In order to clear the CMOS, I had to even take off my HB SLI bridge and bottom video card just to clear the CMOS. OK, so with that out of the way, I have a set of fresh clean settings. Booted up back into Windows with no BIOS changes. Everything seems OK.
Now back to the BIOS because I need to enable XMP on my G.Skill kit, 4x8GB 3200 Mhz. Selected XMP, changed no other settings. Save settings, reboot. Boot failure.
Here are my relevant PC specs.
Intel Core i9 7900x
Asus Prime X299-A
G.Skill 4x8GB, 3200 MHz C16, full Quad Channel Kit, F4-3200C16Q-32GTZR
2x MSI GTX 1080 Gaming X in SLI with MSI SLI HB Bridge
Upon bootup failure, the on-board LED codes were: 11, 52, and 55, on different boot attempts. Those numbers were freeze and boot would not proceed.
I have decided to keep 0503 for now, with the default 2133 MHz RAM setting to see if anyone else has some work-arounds and/or a quick BIOS hotfix will be provided within the next week. The reason I don't want to do diagnostics today (mostly since I am busy with work), but because in order to access the clear CMOS header, I have to remove my second video card every single time. So I am currently uninterested in manually setting RAM values one-at-a-time to diagnose the root cause.
Another annoying thing I have to do each time: delete/recreate my Intel RAID 0 array of SSDs for games, 4x 240 GB drives. Since the settings are lost every time I reset the settings, I not only have to press CTRL+I to setup the array, but also run Testdisk in Windows to recover the volume. Thankfully that tool works so well.
Needless to say, XMP and BIOS 0503 is causing me trouble--big trouble.