Channel overlays while creating systems

Here's my case: I want to use nixpkgs-unstable channel on my main machine, but nixpkgs-stable on my partner's one due to stability reasons. Flake-utils-plus allows that and it's cool. However, I also want to access my own packages inside partner's configuration. The most straightforward way I thought of was like this (inside lib.mkFlake's argument):

      package-namespace = "my";
      channels.nixpkgs-stable.overlaysBuilder = lib.overlay.create-overlays-builder {
        inherit package-namespace;
      };
      systems.hosts.my-partner's-machine.channelName = "nixpkgs-stable";

However, while evaluating the configuration nix shouts:

error: attribute 'my' missing

It feels like the channels.nixpkgs-stable parameter is being completely ignored. That's quite strange, since the same block of code works for default nixpkgs:
https://github.com/snowfallorg/lib/blob/92803a029b5314d4436a8d9311d8707b71d9f0b6/snowfall-lib/flake/default.nix#L173-L176

And I don't see channels parameter removed in snowfall-lib.flake.without-snowfall-options or anywhere else... Strange!

Start with the lib.mkFlake argument block in the issue and compare the channel-specific behavior with flake/default.nix lines 173-176, where the default nixpkgs overlay works. Then inspect snowfall-lib.flake.without-snowfall-options to trace whether channels.nixpkgs-stable.overlaysBuilder is preserved; done means the partner system can access the package namespace from nixpkgs-stable without the missing-attribute error.