Greetings!
As far as I know disabling CSM should lead to installation in UEFI mode, yet I am still seeing GRUB during boot. What's weird, that efibootmgr -v reports that Proxmox were booted in UEFI mode.
root@atlantis:~# efibootmgr -v
BootCurrent: 000C
Timeout: 1 seconds
BootOrder: 000C,0001...