@relief-melone Did you get this figured out? I am having similar trouble with NixOS able to boot into the installer after I change to OtherOS from windows EFI mode, but it will then install and not boot. Sometimes it will shows multiple EFI partitions in my bios boot loader as well. But even when it shows correct partitions, trying to boot takes me immediately back to the BIOS firmware screen.
I’ve tried disabling legacy boot, disabling secure boot and UEFI, but nothing works.
I am on an old ASUS z97-A motherboard with i7-4790k and AMD 5600 GPU