Nixos-rebuild --upgrade removing channels?

I’ve got a strange phenomen on my system that I don’t know where to start investigating.
I have 3 channels configured on my system, two of which I use in my configuration.nix, the current nixos channel (21.05) and nixos-2009.
nix-channel --update correctly updates all of my channels.
Running nixos-rebuild boot --upgrade or any other subcommand with --upgrade results in the nixos-2009 channel mysteriously disappearing and thus my system build failing.

Example:

[root@ashe:/nix/var/nix/profiles/per-user/root]# pwd && echo $NIX_PATH && nix --version && nixos-version && nix-channel --list && nix-channel --update && ls -lF channels/ && nixos-rebuild boot --upgrade --keep-going -j0 ; ls -lF channels/
/nix/var/nix/profiles/per-user/root
/root/.nix-defexpr/channels:nixpkgs=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels
nix (Nix) 2.3.12
21.05.1340.688f6ec8d76 (Okapi)
nixos https://nixos.org/channels/nixos-21.05-small
nixos-2009 https://nixos.org/channels/nixos-20.09-small
nixos-unstable https://nixos.org/channels/nixos-unstable-small
unpacking channels...
insgesamt 2
lrwxrwxrwx 2 root root  60  1. Jan 1970  manifest.nix -> /nix/store/1nawkb4sv5jvkfc43hbcfdjwv9avv8ic-env-manifest.nix
lrwxrwxrwx 3 root root  78  1. Jan 1970  nixos -> /nix/store/0n6l03p8bxyg4agxzmy7528wffnfbjzn-nixos-21.05.1412.cf59fbd5396/nixos/
lrwxrwxrwx 2 root root  88  1. Jan 1970  nixos-2009 -> /nix/store/cf0yxr8c2w373mnc38qsvz2pdib0afr7-nixos-2009-20.09.4393.068984c00e0/nixos-2009/
lrwxrwxrwx 4 root root 100  1. Jan 1970  nixos-unstable -> /nix/store/bfw6vw2i25kxvypzzy07h2r2sbyznw62-nixos-unstable-21.11pre301432.faec8bbbf78/nixos-unstable/
unpacking channels...
building Nix...
building the system configuration...
trace: Warning: `stdenv.lib` is deprecated and will be removed in the next release. Please use `lib` instead. For more information see https://github.com/NixOS/nixpkgs/issues/108938
error: file 'nixos-2009' was not found in the Nix search path (add it using $NIX_PATH or -I), at /etc/nixos/git/ashe-sw.nix:9:23
(use '--show-trace' to show detailed location information)
insgesamt 2
lrwxrwxrwx 2 root root  60  1. Jan 1970  manifest.nix -> /nix/store/xnbf40imsyfqn9b4vghqxvf0b4vc2sp0-env-manifest.nix
lrwxrwxrwx 3 root root  78  1. Jan 1970  nixos -> /nix/store/0n6l03p8bxyg4agxzmy7528wffnfbjzn-nixos-21.05.1412.cf59fbd5396/nixos/
lrwxrwxrwx 4 root root 100  1. Jan 1970  nixos-unstable -> /nix/store/bfw6vw2i25kxvypzzy07h2r2sbyznw62-nixos-unstable-21.11pre301432.faec8bbbf78/nixos-unstable/