Do we know if it was the VBIOS or system BIOS? I think it was the VBIOS from late May? It is video BIOS update for rebar.  Here is the post that summarizes the issue: [URL]  
Hey all Is it just me or did the latest VBIOS/BIOS for the R12 and RTX3090 bork something. I now have ReBAR support but can no longer see the post screen or access BIOS when connected via DisplayPort. All good via HDMI.  I've seen some people reporting similar. On previous models R10 and prior the solution seemed to be to disable secure boot and avoid the UEFI environment but not sure that is possible on R12s. Good to know if anyone else is having same issues. Thanks Nate
Aurora R12, no Post/Bios, DisplayPort after latest VBIOS