Systemd fails to umount /nix/store when installed with LUKs + LVM in a separate partition

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I have the following environment:

❯ lsblk
NAME                 MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
sda                    8:0    0  1.8T  0 disk  
├─sda1                 8:1    0    1G  0 part  /boot
└─sda2                 8:2    0  1.8T  0 part  
  └─root             254:0    0  1.8T  0 crypt 
    ├─nixos--vg-swap 254:1    0   16G  0 lvm   [SWAP]
    ├─nixos--vg-root 254:2    0  200G  0 lvm   /
    ├─nixos--vg-var  254:3    0  200G  0 lvm   /var
    ├─nixos--vg-nix  254:4    0  200G  0 lvm   /nix
    └─nixos--vg-home 254:5    0  1.2T  0 lvm   /home

This the generated hardware-configuration.nix:

# Do not modify this file!  It was generated by ‘nixos-generate-config’
# and may be overwritten by future invocations.  Please make changes
# to /etc/nixos/configuration.nix instead.
{ config, lib, pkgs, modulesPath, ... }:

{
  imports =
    [ (modulesPath + "/installer/scan/not-detected.nix")
    ];

  boot.initrd.availableKernelModules = [ "xhci_pci" "ahci" "usbhid" "usb_storage" "sd_mod" ];
  boot.initrd.kernelModules = [ "dm-snapshot" ];
  boot.kernelModules = [ "kvm-amd" ];
  boot.extraModulePackages = [ ];

  fileSystems."/" =
    { device = "/dev/disk/by-uuid/d5f672ab-6578-4f63-9c01-2798813eba84";
      fsType = "ext4";
    };

  fileSystems."/boot" =
    { device = "/dev/disk/by-uuid/7B9A-3132";
      fsType = "vfat";
    };

  fileSystems."/home" =
    { device = "/dev/disk/by-uuid/5bbf03fa-558a-453d-9554-b2a4e5df3fc2";
      fsType = "ext4";
    };

  fileSystems."/var" =
    { device = "/dev/disk/by-uuid/f246ec0f-3dd9-426c-92a9-3791b4a6e77f";
      fsType = "ext4";
    };

  fileSystems."/nix" =
    { device = "/dev/disk/by-uuid/e6a4daaa-dfc4-4082-aff0-482cf51c0f16";
      fsType = "ext4";
    };

  swapDevices =
    [ { device = "/dev/disk/by-uuid/8d192ac1-7b4d-44ec-b5e1-7dcb685c00ef"; }
    ];

  # high-resolution display
  hardware.video.hidpi.enable = lib.mkDefault true;
}

To ask for password before mouting /dev/sda2 when booting my system:

  boot.initrd.luks.devices = {
    root = {
	  device = "/dev/sda2";
	  preLVM = true;
    };
  };

However, every time I shutdown my system it seems I get error warnings from systemd that it can’t umount /nix/store.

nix-shell -p nix-info --run "nix-info -m"
these paths will be fetched (0.05 MiB download, 0.28 MiB unpacked):
  /nix/store/a9g7p6fwanw66j5djzila7ql1hky759z-bash-interactive-4.4-p23-dev
copying path '/nix/store/a9g7p6fwanw66j5djzila7ql1hky759z-bash-interactive-4.4-p23-dev' from 'https://cache.nixos.org'...
 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.108, NixOS, 20.09.3686.da7f4c48425 (Nightingale)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.10`
 - channels(root): `"nixos-20.09.3728.cadb0637829"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`