My computer often switches the default monitor when I boot, sometimes it's on my 1440p main, sometimes it switches without reason to my 1080p - but once I get to Windows login it goes to my 1440p 100% of the time.
No idea on a fix, though. I played with the default BIOS and XOC BIOS and it had the same issues. Maybe it's just a 1080 Ti quirk?
Good luck figuring something out, I'd be sad if I had to fiddle with cables to fix my issues