i have the same BAR behaviour on my ASUS X299 SAGE/10G:
with BAR support enabled in BIOS, i have half the time on bootprocess verbose logs,
than verbose log stops but machine continues bootprocess and i am able to login via RemoteDesktopClient from another maschine.
But on the ASUS no Monitor Output at all.
No matter if corresponding OC values set to "-1" or any other Value.