File system "/dev/md127" is not located on a partitioned block device.
Traceback (most recent call last):
  File "/nix/store/ks9dxd7yp3pj64ib7y19pgbgsnir9xgm-systemd-boot/bin/systemd-boot", line 431, in <module>
    main()
  File "/nix/store/ks9dxd7yp3pj64ib7y19pgbgsnir9xgm-systemd-boot/bin/systemd-boot", line 414, in main
    install_bootloader(args)
  File "/nix/store/ks9dxd7yp3pj64ib7y19pgbgsnir9xgm-systemd-boot/bin/systemd-boot", line 307, in install_bootloader
    run(
  File "/nix/store/ks9dxd7yp3pj64ib7y19pgbgsnir9xgm-systemd-boot/bin/systemd-boot", line 58, in run
    return subprocess.run(cmd, check=True, text=True, stdout=stdout)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/bbyp6vkdszn6a14gqnfx8l5j3mhfcnfs-python3-3.12.11/lib/python3.12/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/nix/store/ksz6ygnl9f1l6jff3mwmjsmw7qyyxa59-systemd-257.7/bin/bootctl', '--esp-path=/boot', 'install']' returned non-zero exit status 1.
Failed to install bootloader

you could check this:

       If $SYSTEMD_RELAX_ESP_CHECKS=1 is set the validation checks for
       the ESP are relaxed, and the path specified with --esp-path= may
       refer to any kind of file system on any kind of partition.

       Similarly, $SYSTEMD_RELAX_XBOOTLDR_CHECKS=1 turns off some
       validation checks for the Extended Boot Loader partition.

environment.systemPackages = [
(pkgs.writeScriptBin "nixos-install" ''
  export SYSTEMD_RELAX_ESP_CHECKS=1
  exec {pkgs.nixos-install}/bin/nixos-install "$@"
'')
]

https://github.com/systemd/systemd/issues/12468#issuecomment-698782077

And yes, sd-boot writes to the ESP if boot counting/boot assessment is enabled.